As filed with the Securities and Exchange Commission on April 30, 1999.
                              Registration No. 333-
       -----------------------------------------------------------------

                       SECURITIES AND EXCHANGE COMMISSION

                             WASHINGTON, D.C. 20549
                                 ---------------

                                    FORM S-8
                             Registration Statement
                                      Under
                           The Securities Act of 1933
                                 ---------------

                                 THERMEDICS INC.
             (Exact name of registrant as specified in its charter)
                                 ---------------

      DELAWARE                                             04-2788806
(State or other jurisdiction of                       (I.R.S. Employer
 incorporation or organization)                 Identification Number)

                               470 Wildwood Street
                        Woburn, Massachusetts 01888-1799
               (Address of Principal Executive Offices) (Zip Code)


                 THERMEDICS INC. EMPLOYEES EQUITY INCENTIVE PLAN

            THERMEDICS INC. DEFERRED COMPENSATION PLAN FOR DIRECTORS

        THERMO VOLTEK CORP. NONQUALIFIED AND INCENTIVE STOCK OPTION PLAN

                    THERMO VOLTEK CORP. EQUITY INCENTIVE PLAN

                 THERMO VOLTEK CORP. DIRECTORS STOCK OPTION PLAN

      THERMEDICS INC. / THERMO VOLTEK CORP. NONQUALIFIED STOCK OPTION PLAN

                   THERMEDICS INC. DIRECTORS STOCK OPTION PLAN

THERMO ELECTRON CORPORATION / THERMO VOLTEK CORP. NONQUALIFIED STOCK OPTION PLAN

             THERMO ELECTRON CORPORATION DIRECTORS STOCK OPTION PLAN

  THERMO ELECTRON CORPORATION / THERMEDICS INC. NONQUALIFIED STOCK OPTION PLAN

                           (Full Titles of the Plans)

<PAGE>


                            Sandra L. Lambert, Clerk
                                 Thermedics Inc.
                         c/o Thermo Electron Corporation
                                 81 Wyman Street
                                 P. O. Box 9046
                        Waltham, Massachusetts 02454-9046
                     (Name and Address of Agent for Service)

                                 (781) 622-1000
          (Telephone Number, Including Area Code, of Agent For Service)

                                   Copies to:
                    Seth H. Hoogasian, Esq., General Counsel
                                 Thermedics Inc.
                         c/o Thermo Electron Corporation
                                 81 Wyman Street
                                 P. O. Box 9046
                        Waltham, Massachusetts 02454-9046

                                 ---------------

                         CALCULATION OF REGISTRATION FEE


Title of                                     Proposed 
securities          Amount       Proposed     Maximum              Amount of
to be               to be        Maximum     Aggregate           Registration 
registered        registered    Offering     Offering Price         Fee
                                Price Per
                                  Share
Common Stock,
$.10 par          1,294,350     $7.03125     $9,100,898.44        $2,531
value per         shares (1)       (2)            (2)
share       


      In  addition,  pursuant to Rule 416(c) under the  Securities  Act of 1933,
this Registration  Statement also covers an indeterminate amount of interests to
be offered or sold pursuant to the employee  benefit plans described  herein and
an  indeterminate  number of shares of the  Registrant's  Common Stock as may be
issuable  in  connection  with  adjustments  under the  employee  benefit  plans
described  herein  to  reflect  certain  changes  in  the  Registrant's  capital
structure, including stock dividends or stock splits.
<PAGE>

(1)   The shares registered hereunder are divided among the various plans as set
      forth in the following table:

      Name of Plan                                          No. of Shares

      Thermedics Inc.
            Employees Equity Incentive Plan                      300,000

      Thermedics Inc. Deferred Compensation Plan                  30,000
            For Directors

      Thermo Voltek Corp. Nonqualified and Incentive             298,300
            Stock Option Plan

      Thermo Voltek Corp. Equity Incentive Plan                  295,500

      Thermo Voltek Corp. Directors Stock Option Plan             15,150

      Thermedics Inc./Thermo Voltek Corp. Nonqualified             5,000
            Stock Option Plan

      Thermedics Inc. Directors Stock Option Plan                  6,750

      Thermo Electron Corporation/Thermo Voltek Corp.             33,150
            Nonqualified Stock Option Plan

      Thermo Electron Corporation Directors Stock                160,500
            Option Plan

      Thermo Electron Corporation/Thermedics Inc.                150,000
            Nonqualified Stock Option Plan


(2)   Estimated  solely  for  the  purpose  of  calculating  the  amount  of the
      registration  fee in accordance  with Rule 457(h) under the Securities Act
      of 1933. The calculation of the proposed maximum aggregate  offering price
      has been based upon (1) the  registration  hereunder  of an  aggregate  of
      1,294,350 shares and (2) the average of the high and low sales prices,  $7
      3/16 and $6 7/8,  respectively,  of the  Registrant's  Common Stock on the
      American Stock Exchange on April 27, 1999 as reported in the  consolidated
      reporting system.



<PAGE>




                                EXPLANATORY NOTE

      With  respect  to  the  Thermo  Electron  Corporation  /  Thermedics  Inc.
Nonqualified Stock Option Plan (the "TMO/TMD Plan"), this Registration Statement
is filed  pursuant  to General  Instruction  E of Form S-8 in order to  register
additional  securities  of the  same  class  as  other  securities  for  which a
registration statement relating to such employee benefit plan is effective.

      On October 11, 1984, the Company filed an initial  registration  statement
on Form S-8 (Reg.  No.  2-93746)  to  register  83,550  shares  of Common  Stock
reserved for issuance under the TMO/TMD Plan.  Giving effect to increases  under
the TMO/TMD Plan as a result of stock splits,  the addition of this Registration
Statement  brings the total number of shares  registered  for issuance under the
TMO/TMD  Plan to 463,312.  The  contents of the  previously  filed  registration
statement  relating to the TMO/TMD  Plan are  incorporated  herein by  reference
except for the items that are set forth below.  Pursuant to  Instruction E, this
Registration  Statement  contains such information  required by Form S-8 that is
not otherwise included in the previous registration statement.



                                     PART I

              INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

      The information  required by Part I is included in documents sent or given
to the  respective  participants  in the plans  listed on the cover page of this
Registration  Statement  pursuant to Rule 428(b)(1)  under the Securities Act of
1933, as amended (the "Securities Act"). As used in this Registration Statement,
the terms "Registrant" or "Company" refer to Thermedics Inc.


                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.  Incorporation of Documents by Reference.

      The Registrant is subject to the informational and reporting  requirements
of Sections 13(a),  13(c), 14 and 15(d) of the Securities  Exchange Act of 1934,
as amended (the  "Exchange  Act"),  and in accordance  therewith  files reports,
proxy  statements  and  other  information  with  the  Securities  and  Exchange
Commission (the "Commission").  The following documents,  which are on file with
the Commission, are incorporated in this Registration Statement by reference:

            (a)   The  Company's  Annual Report on Form 10-K for the fiscal year
                  ended January 2, 1999; and
<PAGE>

            (b)   The  description of the Common Stock which is contained in the
                  Company's  Registration  Statement on Form 8-A filed under the
                  Exchange Act, as such  description may be amended from time to
                  time.

      All reports or proxy  statements filed by the Company pursuant to Sections
13(a),  13(c),  14 and 15(d) of the Exchange Act  subsequent to the date of this
Registration  Statement  and prior to the filing of a  post-effective  amendment
that  indicates  that all  securities  offered  herein  have been sold,  or that
deregisters  all such securities  then remaining  unsold,  shall be deemed to be
incorporated by reference in this Registration Statement and to be a part hereof
from the respective dates of filing of such documents.

Item 4.  Description of Securities.

      Not applicable.

Item 5.  Interests of Named Experts and Counsel.

      The  validity of the Common Stock  offered  hereby has been passed upon by
Seth H.  Hoogasian,  Esq.,  General Counsel of the Company.  Mr.  Hoogasian is a
full-time employee of Thermo Electron Corporation ("Thermo Electron"),  which is
the majority stockholder of the Company, is an officer of the Company and Thermo
Electron,  and owns or has the right to acquire 9,565 shares of Common Stock and
348,828  shares of the  common  stock,  $1.00 par  value  per  share,  of Thermo
Electron.

Item 6.  Indemnification of Directors and Officers.

      The Massachusetts  Business  Corporation Law and the Company's Articles of
Incorporation  and By-Laws  limit the  monetary  liability  of  directors to the
Company and to its stockholders and provide for indemnification of the Company's
officers and directors for  liabilities and expenses that they may incur in such
capacities.  In general,  officers and directors are indemnified with respect to
actions  taken in good faith in a manner  reasonably  believed  to be in, or not
opposed to, the best  interests of the Company and, with respect to any criminal
action or proceeding,  actions that the  indemnitee  had no reasonable  cause to
believe were unlawful. The Company also has indemnification  agreements with its
directors and officers that provide for the maximum  indemnification  allowed by
law.

      Thermo  Electron has an insurance  policy which  insures the directors and
officers of Thermo Electron and its subsidiaries, including the Company, against
certain  liabilities  which might be incurred in connection with the performance
of their duties.
<PAGE>


Item 7.  Exemption from Registration Claimed.

      Not Applicable.

Item 8.  Exhibits.

      The Exhibit Index  immediately  preceding the exhibits is attached  hereto
and incorporated herein by reference.

Item 9.  Undertakings.

      (a)   The undersigned Registrant hereby undertakes:

            (1) To file,  during any  period in which  offers or sales are being
made, a post-effective amendment to this registration statement:

                  (i)    To include any prospectus required by Section
     10(a)(3) of the Securities Act of 1933;

                  (ii) To reflect in the  prospectus any facts or events arising
      after the effective date of the registration statement (or the most recent
      post-effective amendment thereof) which, individually or in the aggregate,
      represent  a  fundamental  change  in the  information  set  forth  in the
      registration  statement.  Notwithstanding  the foregoing,  any increase or
      decrease in volume of  securities  offered (if the total  dollar  value of
      securities  offered  would not exceed that which was  registered)  and any
      deviation from the low or high end of the estimated maximum offering range
      may be  reflected  in the form of  prospectus  filed  with the  Commission
      pursuant  to Rule 424(b) if, in the  aggregate,  the changes in volume and
      price  represent no more than 20 percent  change in the maximum  aggregate
      offering price set forth in the "Calculation of Registration Fee" table in
      the effective registration statement;

                  (iii) To include any material  information with respect to the
      plan  of  distribution  not  previously   disclosed  in  the  registration
      statement or any material change to such  information in the  registration
      statement.

      Provided,  however,  that paragraphs (a)(1)(i) and (a)(1)(ii) do not apply
if the  information  required to be included in a  post-effective  amendment  by
those paragraphs is contained in periodic reports filed with or furnished to the
Commission  by the  Registrant  pursuant  to Section 13 or Section  15(d) of the
Securities  Exchange  Act of 1934  that are  incorporated  by  reference  in the
registration statement.
<PAGE>

            (2) That,  for the purpose of  determining  any liability  under the
Securities Act of 1933, each such post-effective amendment shall be deemed to be
a new registration statement relating to the securities offered therein, and the
offering of such  securities at that time shall be deemed to be the initial bona
fide offering thereof.

            (3)  To  remove  from  registration  by  means  of a  post-effective
amendment  any of the  securities  being  registered  which remain unsold at the
termination of the offering.

      (b) The undersigned  Registrant  hereby  undertakes  that, for purposes of
determining  any liability  under the Securities Act of 1933, each filing of the
Registrant's  annual report pursuant to Section 13(a) or 15(d) of the Securities
Exchange Act of 1934 (and, where applicable,  each filing of an employee benefit
plan's annual report pursuant to Section 15(d) of the Securities Exchange Act of
1934) that is incorporated by reference in the  registration  statement shall be
deemed to be a new  registration  statement  relating to the securities  offered
therein,  and the offering of such securities at that time shall be deemed to be
the initial bona fide offering thereof.

      (c)  Insofar  as  indemnification   for  liabilities   arising  under  the
Securities Act of 1933 may be permitted to directors,  officers and  controlling
persons of the Registrant  pursuant to the foregoing  provisions,  or otherwise,
the  Registrant  has been  advised  that in the  opinion of the  Securities  and
Exchange  Commission such  indemnification is against public policy as expressed
in the  Securities  Act and is,  therefore,  unenforceable.  In the event that a
claim for  indemnification  against such liabilities  (other than the payment by
the  Registrant  of  expenses  incurred  or  paid  by  a  director,  officer  or
controlling  person of the Registrant in the  successful  defense of any action,
suit or proceeding) is asserted by such director,  officer or controlling person
in connection with the securities being registered,  the Registrant will, unless
in the  opinion  of its  counsel  the matter  has been  settled  by  controlling
precedent,  submit to a court of appropriate  jurisdiction  the question whether
such  indemnification  by it is  against  public  policy  as  expressed  in  the
Securities Act and will be governed by the final adjudication of such issue.


<PAGE>


                                   SIGNATURES

      Pursuant to the requirements of the Securities Act of 1933, the Registrant
certifies  that it has  reasonable  grounds to believe  that it meets all of the
requirements  for  filing  on Form S-8 and has  duly  caused  this  Registration
Statement  to be  signed  on its  behalf  by  the  undersigned,  thereunto  duly
authorized, in the City of Woburn,  Commonwealth of Massachusetts,  on this 30th
day of April, 1999.

                                 THERMEDICS INC.


                                    By: /s/ John T. Keiser
                                        ---------------------------------------
                                        John T. Keiser
                                        President and
                                        Chief Executive Officer


                                POWER OF ATTORNEY

      Each of the  undersigned  Directors and Officers of Thermedics Inc. hereby
appoints Theo  Melas-Kyriazi,  Paul F. Kelleher,  Kenneth J.  Apicerno,  Seth H.
Hoogasian  and  Sandra  L.  Lambert,  and  each of them,  his  true  and  lawful
attorneys-in-fact  and agents,  with full power of substitution,  for him and in
his  name,  place  and  stead,  in any and all  capacities,  to sign any and all
amendments (including post-effective amendments) to this Registration Statement,
and to file the same, with all exhibits thereto, and all documents in connection
therewith,  with the  Securities  and Exchange  Commission,  granting  unto said
attorneys-in-fact  and agents,  and each of them, full power and authority to do
and perform each and every act and thing  requisite  and necessary to be done in
and about the  premises,  as fully to all  intents  and  purposes as he might or
could  do  in  person,   hereby   ratifying   and   confirming   all  that  said
attorneys-in-fact  and  agents  or any of them,  or their or his  substitute  or
substitutes, may lawfully do or cause to be done by virtue thereof.

      Pursuant  to  the  requirements  of  the  Securities  Act  of  1933,  this
Registration  Statement  has been signed below by the  following  persons in the
capacities and on the dates indicated:

Signature                          Title                         Date

                                                                
/s/ John T. Keiser            President, Chief Executive    April 30, 1999
-------------------------     Officer and Director
John T. Keiser

/s/ Theo Melas-Kyriazi
-------------------------     Chief Financial Officer       April 30, 1999
Theo Melas-Kyriazi                          

/s/ Paul F. Kelleher          Chief Accounting Officer      April 30, 1999
-------------------------
Paul F. Kelleher

/s/ John W. Wood Jr.          Chairman of the Board of      April 30, 1999
-------------------------     Directors
John W. Wood Jr.

/s/ T. Anthony Brooks         Director                      April 30, 1999
-------------------------
T. Anthony Brooks

/s/ Peter O. Crisp            Director                      April 30, 1999
-------------------------
Peter O. Crisp


-------------------------     Director                      April 30, 1999
Paul F. Ferrari

/s/ George N. Hatsopoulos     Director                      April 30, 1999
-------------------------
George N. Hatsopoulos

/s/ John N. Hatsopoulos       Director                      April 30, 1999
-------------------------
John N. Hatsopoulos

/s/ Nicholas T. Zervas        Director                      April 30, 1999
-------------------------
Nicholas T. Zervas




<PAGE>


EXHIBIT INDEX


Exhibit
Number            Description


5.1               Opinion of Seth H. Hoogasian, Esq.

23.1              Consent of Arthur Andersen LLP

23.2              Consent of Seth H. Hoogasian, Esq.
                  (contained in his opinion filed as
                  Exhibit 5.1).

24                Power of Attorney (see  signature  pages to this  Registration
                  Statement).



<PAGE>





                                                                 EXHIBIT 5.1



                                 Thermedics Inc.
                               470 Wildwood Street
                        Woburn, Massachusetts 01888-1799




                                                  April 30, 1999

Thermedics Inc.
470 Wildwood Street
Woburn, Massachusetts 01888-1799


Re:   Registration Statement on Form S-8
      Relating to 1,294,350 Shares of the Common Stock,
      $.10 par value, of Thermedics Inc.

Dear Sirs:

     I am General Counsel to Thermedics  Inc., a Massachusetts  corporation (the
"Company"),  and have acted as counsel in connection with the registration under
the  Securities  Act of  1933,  as  amended,  on  Form  S-8  (the  "Registration
Statement"),  of 1,294,350 shares of the Company's Common Stock,  $.10 par value
per share (the "Shares") subject to the following  employee benefit plans (each,
a "Plan"):  Thermedics Inc.  Employees  Equity  Incentive Plan;  Thermedics Inc.
Deferred Compensation Plan for Directors;  Thermo Voltek Corp.  Nonqualified and
Incentive Stock Option Plan;  Thermo Voltek Corp.  Equity Incentive Plan; Thermo
Voltek Corp.  Directors Stock Option Plan;  Thermedics  Inc./Thermo Voltek Corp.
Nonqualified  Stock Option Plan;  Thermedics  Inc.  Directors Stock Option Plan;
Thermo Electron  Corporation/Thermo Voltek Corp. Nonqualified Stock Option Plan;
Thermo  Electron  Corporation  Directors  Stock Option Plan; and Thermo Electron
Corporation/Thermedics Inc. Nonqualified Stock Option Plan.

      I or a member of my legal staff have  reviewed the  corporate  proceedings
taken by the Company  with respect to the  authorization  of the issuance of the
Shares.  I or a member of my legal  staff have also  examined  and  relied  upon
originals or copies, certified or otherwise authenticated to my satisfaction, of
all corporate records, documents, agreements or other instruments of the Company
and have made all  investigations  of law and have  discussed with the Company's
representatives   all  questions  of  fact  that  I  have  deemed  necessary  or
appropriate.

<PAGE>

      Based upon and subject to the foregoing, I am of the opinion that:

      1. The Company is a corporation  duly organized,  validly  existing and in
corporate good standing under the laws of the Commonwealth of Massachusetts.

      2. The issuance and sale of the Shares as contemplated in the Registration
Statement have been duly authorized by the Company.

      3. The Shares,  when issued and sold in accordance  with the provisions of
the applicable Plan, will be validly issued, fully paid and nonassessable.

      I hereby  consent  to the filing of this  opinion  as  Exhibit  5.1 to the
Registration Statement.

                                          Very truly yours,


                                          /s/ Seth H. Hoogasian
                                          Seth H. Hoogasian
                                          General Counsel


<PAGE>




                                                                 Exhibit 23.1






                    Consent of Independent Public Accountants
                    -----------------------------------------


     As independent public  accountants,  we hereby consent to the incorporation
by  reference  in this  Registration  Statement  on Form S-8 of our report dated
February 11, 1999,  included or incorporated  by reference in Thermedics  Inc.'s
Annual  Report on Form  10-K for the year  ended  January  2,  1999,  and to all
references to our Firm included in this Registration Statement.


                                                  ARTHUR ANDERSEN LLP



Boston, Massachusetts
April 28, 1999
[{"type": "S-8", "sequence": "1", "description": "FORM S-8", "secsgml_size_bytes": 23311}]
{"company-data": {"conformed-name": "THERMEDICS INC", "cik": "0000721356", "assigned-sic": "3590", "irs-number": "042788806", "state-of-incorporation": "DE", "fiscal-year-end": "0102"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-77487", "film-number": "99607465"}, "business-address": {"street1": "470 WILDWOOD ST", "street2": "P O BOX 2697", "city": "WOBURN", "state": "MA", "zip": "01888-1799", "phone": "7819383786"}, "mail-address": {"street1": "81 WYMAN STREET", "street2": "P.O. BOX 9046", "city": "WALTHAM", "state": "MA", "zip": "02254"}}
As filed with the Securities and Exchange Commission on April 30, 1999.
                              Registration No. 333-
       -----------------------------------------------------------------

                       SECURITIES AND EXCHANGE COMMISSION

                             WASHINGTON, D.C. 20549
                                 ---------------

                                    FORM S-8
                             Registration Statement
                                      Under
                           The Securities Act of 1933
                                 ---------------

                                 THERMEDICS INC.
             (Exact name of registrant as specified in its charter)
                                 ---------------

      DELAWARE                                             04-2788806
(State or other jurisdiction of                       (I.R.S. Employer
 incorporation or organization)                 Identification Number)

                               470 Wildwood Street
                        Woburn, Massachusetts 01888-1799
               (Address of Principal Executive Offices) (Zip Code)

                 THERMEDICS INC. EMPLOYEES EQUITY INCENTIVE PLAN

            THERMEDICS INC. DEFERRED COMPENSATION PLAN FOR DIRECTORS

        THERMO VOLTEK CORP. NONQUALIFIED AND INCENTIVE STOCK OPTION PLAN

                    THERMO VOLTEK CORP. EQUITY INCENTIVE PLAN

                 THERMO VOLTEK CORP. DIRECTORS STOCK OPTION PLAN

      THERMEDICS INC. / THERMO VOLTEK CORP. NONQUALIFIED STOCK OPTION PLAN

                   THERMEDICS INC. DIRECTORS STOCK OPTION PLAN

THERMO ELECTRON CORPORATION / THERMO VOLTEK CORP. NONQUALIFIED STOCK OPTION PLAN

             THERMO ELECTRON CORPORATION DIRECTORS STOCK OPTION PLAN

  THERMO ELECTRON CORPORATION / THERMEDICS INC. NONQUALIFIED STOCK OPTION PLAN

                           (Full Titles of the Plans)
Sandra L. Lambert, Clerk
                                 Thermedics Inc.
                         c/o Thermo Electron Corporation
                                 81 Wyman Street
                                 P. O. Box 9046
                        Waltham, Massachusetts 02454-9046
                     (Name and Address of Agent for Service)

                                 (781) 622-1000
          (Telephone Number, Including Area Code, of Agent For Service)

                                   Copies to:
                    Seth H. Hoogasian, Esq., General Counsel
                                 Thermedics Inc.
                         c/o Thermo Electron Corporation
                                 81 Wyman Street
                                 P. O. Box 9046
                        Waltham, Massachusetts 02454-9046

                                 ---------------

                         CALCULATION OF REGISTRATION FEE

Title of                                     Proposed 
securities          Amount       Proposed     Maximum              Amount of
to be               to be        Maximum     Aggregate           Registration 
registered        registered    Offering     Offering Price         Fee
                                Price Per
                                  Share
Common Stock,
$.10 par          1,294,350     $7.03125     $9,100,898.44        $2,531
value per         shares (1)       (2)            (2)
share       

      In  addition,  pursuant to Rule 416(c) under the  Securities  Act of 1933,
this Registration  Statement also covers an indeterminate amount of interests to
be offered or sold pursuant to the employee  benefit plans described  herein and
an  indeterminate  number of shares of the  Registrant's  Common Stock as may be
issuable  in  connection  with  adjustments  under the  employee  benefit  plans
described  herein  to  reflect  certain  changes  in  the  Registrant's  capital
structure, including stock dividends or stock splits.
(1)   The shares registered hereunder are divided among the various plans as set
      forth in the following table:

      Name of Plan                                          No. of Shares

      Thermedics Inc.
            Employees Equity Incentive Plan                      300,000

      Thermedics Inc. Deferred Compensation Plan                  30,000
            For Directors

      Thermo Voltek Corp. Nonqualified and Incentive             298,300
            Stock Option Plan

      Thermo Voltek Corp. Equity Incentive Plan                  295,500

      Thermo Voltek Corp. Directors Stock Option Plan             15,150

      Thermedics Inc./Thermo Voltek Corp. Nonqualified             5,000
            Stock Option Plan

      Thermedics Inc. Directors Stock Option Plan                  6,750

      Thermo Electron Corporation/Thermo Voltek Corp.             33,150
            Nonqualified Stock Option Plan

      Thermo Electron Corporation Directors Stock                160,500
            Option Plan

      Thermo Electron Corporation/Thermedics Inc.                150,000
            Nonqualified Stock Option Plan

(2)   Estimated  solely  for  the  purpose  of  calculating  the  amount  of the
      registration  fee in accordance  with Rule 457(h) under the Securities Act
      of 1933. The calculation of the proposed maximum aggregate  offering price
      has been based upon (1) the  registration  hereunder  of an  aggregate  of
      1,294,350 shares and (2) the average of the high and low sales prices,  $7
      3/16 and $6 7/8,  respectively,  of the  Registrant's  Common Stock on the
      American Stock Exchange on April 27, 1999 as reported in the  consolidated
      reporting system.
EXPLANATORY NOTE

      With  respect  to  the  Thermo  Electron  Corporation  /  Thermedics  Inc.
Nonqualified Stock Option Plan (the "TMO/TMD Plan"), this Registration Statement
is filed  pursuant  to General  Instruction  E of Form S-8 in order to  register
additional  securities  of the  same  class  as  other  securities  for  which a
registration statement relating to such employee benefit plan is effective.

      On October 11, 1984, the Company filed an initial  registration  statement
on Form S-8 (Reg.  No.  2-93746)  to  register  83,550  shares  of Common  Stock
reserved for issuance under the TMO/TMD Plan.  Giving effect to increases  under
the TMO/TMD Plan as a result of stock splits,  the addition of this Registration
Statement  brings the total number of shares  registered  for issuance under the
TMO/TMD  Plan to 463,312.  The  contents of the  previously  filed  registration
statement  relating to the TMO/TMD  Plan are  incorporated  herein by  reference
except for the items that are set forth below.  Pursuant to  Instruction E, this
Registration  Statement  contains such information  required by Form S-8 that is
not otherwise included in the previous registration statement.

                                     PART I

              INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

      The information  required by Part I is included in documents sent or given
to the  respective  participants  in the plans  listed on the cover page of this
Registration  Statement  pursuant to Rule 428(b)(1)  under the Securities Act of
1933, as amended (the "Securities Act"). As used in this Registration Statement,
the terms "Registrant" or "Company" refer to Thermedics Inc.

                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.  Incorporation of Documents by Reference.

      The Registrant is subject to the informational and reporting  requirements
of Sections 13(a),  13(c), 14 and 15(d) of the Securities  Exchange Act of 1934,
as amended (the  "Exchange  Act"),  and in accordance  therewith  files reports,
proxy  statements  and  other  information  with  the  Securities  and  Exchange
Commission (the "Commission").  The following documents,  which are on file with
the Commission, are incorporated in this Registration Statement by reference:

            (a)   The  Company's  Annual Report on Form 10-K for the fiscal year
                  ended January 2, 1999; and
(b)   The  description of the Common Stock which is contained in the
                  Company's  Registration  Statement on Form 8-A filed under the
                  Exchange Act, as such  description may be amended from time to
                  time.

      All reports or proxy  statements filed by the Company pursuant to Sections
13(a),  13(c),  14 and 15(d) of the Exchange Act  subsequent to the date of this
Registration  Statement  and prior to the filing of a  post-effective  amendment
that  indicates  that all  securities  offered  herein  have been sold,  or that
deregisters  all such securities  then remaining  unsold,  shall be deemed to be
incorporated by reference in this Registration Statement and to be a part hereof
from the respective dates of filing of such documents.

Item 4.  Description of Securities.

      Not applicable.

Item 5.  Interests of Named Experts and Counsel.

      The  validity of the Common Stock  offered  hereby has been passed upon by
Seth H.  Hoogasian,  Esq.,  General Counsel of the Company.  Mr.  Hoogasian is a
full-time employee of Thermo Electron Corporation ("Thermo Electron"),  which is
the majority stockholder of the Company, is an officer of the Company and Thermo
Electron,  and owns or has the right to acquire 9,565 shares of Common Stock and
348,828  shares of the  common  stock,  $1.00 par  value  per  share,  of Thermo
Electron.

Item 6.  Indemnification of Directors and Officers.

      The Massachusetts  Business  Corporation Law and the Company's Articles of
Incorporation  and By-Laws  limit the  monetary  liability  of  directors to the
Company and to its stockholders and provide for indemnification of the Company's
officers and directors for  liabilities and expenses that they may incur in such
capacities.  In general,  officers and directors are indemnified with respect to
actions  taken in good faith in a manner  reasonably  believed  to be in, or not
opposed to, the best  interests of the Company and, with respect to any criminal
action or proceeding,  actions that the  indemnitee  had no reasonable  cause to
believe were unlawful. The Company also has indemnification  agreements with its
directors and officers that provide for the maximum  indemnification  allowed by
law.

      Thermo  Electron has an insurance  policy which  insures the directors and
officers of Thermo Electron and its subsidiaries, including the Company, against
certain  liabilities  which might be incurred in connection with the performance
of their duties.
Item 7.  Exemption from Registration Claimed.

      Not Applicable.

Item 8.  Exhibits.

      The Exhibit Index  immediately  preceding the exhibits is attached  hereto
and incorporated herein by reference.

Item 9.  Undertakings.

      (a)   The undersigned Registrant hereby undertakes:

            (1) To file,  during any  period in which  offers or sales are being
made, a post-effective amendment to this registration statement:

                  (i)    To include any prospectus required by Section
     10(a)(3) of the Securities Act of 1933;

                  (ii) To reflect in the  prospectus any facts or events arising
      after the effective date of the registration statement (or the most recent
      post-effective amendment thereof) which, individually or in the aggregate,
      represent  a  fundamental  change  in the  information  set  forth  in the
      registration  statement.  Notwithstanding  the foregoing,  any increase or
      decrease in volume of  securities  offered (if the total  dollar  value of
      securities  offered  would not exceed that which was  registered)  and any
      deviation from the low or high end of the estimated maximum offering range
      may be  reflected  in the form of  prospectus  filed  with the  Commission
      pursuant  to Rule 424(b) if, in the  aggregate,  the changes in volume and
      price  represent no more than 20 percent  change in the maximum  aggregate
      offering price set forth in the "Calculation of Registration Fee" table in
      the effective registration statement;

                  (iii) To include any material  information with respect to the
      plan  of  distribution  not  previously   disclosed  in  the  registration
      statement or any material change to such  information in the  registration
      statement.

      Provided,  however,  that paragraphs (a)(1)(i) and (a)(1)(ii) do not apply
if the  information  required to be included in a  post-effective  amendment  by
those paragraphs is contained in periodic reports filed with or furnished to the
Commission  by the  Registrant  pursuant  to Section 13 or Section  15(d) of the
Securities  Exchange  Act of 1934  that are  incorporated  by  reference  in the
registration statement.
(2) That,  for the purpose of  determining  any liability  under the
Securities Act of 1933, each such post-effective amendment shall be deemed to be
a new registration statement relating to the securities offered therein, and the
offering of such  securities at that time shall be deemed to be the initial bona
fide offering thereof.

            (3)  To  remove  from  registration  by  means  of a  post-effective
amendment  any of the  securities  being  registered  which remain unsold at the
termination of the offering.

      (b) The undersigned  Registrant  hereby  undertakes  that, for purposes of
determining  any liability  under the Securities Act of 1933, each filing of the
Registrant's  annual report pursuant to Section 13(a) or 15(d) of the Securities
Exchange Act of 1934 (and, where applicable,  each filing of an employee benefit
plan's annual report pursuant to Section 15(d) of the Securities Exchange Act of
1934) that is incorporated by reference in the  registration  statement shall be
deemed to be a new  registration  statement  relating to the securities  offered
therein,  and the offering of such securities at that time shall be deemed to be
the initial bona fide offering thereof.

      (c)  Insofar  as  indemnification   for  liabilities   arising  under  the
Securities Act of 1933 may be permitted to directors,  officers and  controlling
persons of the Registrant  pursuant to the foregoing  provisions,  or otherwise,
the  Registrant  has been  advised  that in the  opinion of the  Securities  and
Exchange  Commission such  indemnification is against public policy as expressed
in the  Securities  Act and is,  therefore,  unenforceable.  In the event that a
claim for  indemnification  against such liabilities  (other than the payment by
the  Registrant  of  expenses  incurred  or  paid  by  a  director,  officer  or
controlling  person of the Registrant in the  successful  defense of any action,
suit or proceeding) is asserted by such director,  officer or controlling person
in connection with the securities being registered,  the Registrant will, unless
in the  opinion  of its  counsel  the matter  has been  settled  by  controlling
precedent,  submit to a court of appropriate  jurisdiction  the question whether
such  indemnification  by it is  against  public  policy  as  expressed  in  the
Securities Act and will be governed by the final adjudication of such issue.
SIGNATURES

      Pursuant to the requirements of the Securities Act of 1933, the Registrant
certifies  that it has  reasonable  grounds to believe  that it meets all of the
requirements  for  filing  on Form S-8 and has  duly  caused  this  Registration
Statement  to be  signed  on its  behalf  by  the  undersigned,  thereunto  duly
authorized, in the City of Woburn,  Commonwealth of Massachusetts,  on this 30th
day of April, 1999.

                                 THERMEDICS INC.

                                    By: /s/ John T. Keiser
                                        ---------------------------------------
                                        John T. Keiser
                                        President and
                                        Chief Executive Officer

                                POWER OF ATTORNEY

      Each of the  undersigned  Directors and Officers of Thermedics Inc. hereby
appoints Theo  Melas-Kyriazi,  Paul F. Kelleher,  Kenneth J.  Apicerno,  Seth H.
Hoogasian  and  Sandra  L.  Lambert,  and  each of them,  his  true  and  lawful
attorneys-in-fact  and agents,  with full power of substitution,  for him and in
his  name,  place  and  stead,  in any and all  capacities,  to sign any and all
amendments (including post-effective amendments) to this Registration Statement,
and to file the same, with all exhibits thereto, and all documents in connection
therewith,  with the  Securities  and Exchange  Commission,  granting  unto said
attorneys-in-fact  and agents,  and each of them, full power and authority to do
and perform each and every act and thing  requisite  and necessary to be done in
and about the  premises,  as fully to all  intents  and  purposes as he might or
could  do  in  person,   hereby   ratifying   and   confirming   all  that  said
attorneys-in-fact  and  agents  or any of them,  or their or his  substitute  or
substitutes, may lawfully do or cause to be done by virtue thereof.

      Pursuant  to  the  requirements  of  the  Securities  Act  of  1933,  this
Registration  Statement  has been signed below by the  following  persons in the
capacities and on the dates indicated:

Signature                          Title                         Date

                                                                
/s/ John T. Keiser            President, Chief Executive    April 30, 1999
-------------------------     Officer and Director
John T. Keiser

/s/ Theo Melas-Kyriazi
-------------------------     Chief Financial Officer       April 30, 1999
Theo Melas-Kyriazi                          

/s/ Paul F. Kelleher          Chief Accounting Officer      April 30, 1999
-------------------------
Paul F. Kelleher

/s/ John W. Wood Jr.          Chairman of the Board of      April 30, 1999
-------------------------     Directors
John W. Wood Jr.

/s/ T. Anthony Brooks         Director                      April 30, 1999
-------------------------
T. Anthony Brooks

/s/ Peter O. Crisp            Director                      April 30, 1999
-------------------------
Peter O. Crisp

-------------------------     Director                      April 30, 1999
Paul F. Ferrari

/s/ George N. Hatsopoulos     Director                      April 30, 1999
-------------------------
George N. Hatsopoulos

/s/ John N. Hatsopoulos       Director                      April 30, 1999
-------------------------
John N. Hatsopoulos

/s/ Nicholas T. Zervas        Director                      April 30, 1999
-------------------------
Nicholas T. Zervas
EXHIBIT INDEX

Exhibit
Number            Description

5.1               Opinion of Seth H. Hoogasian, Esq.

23.1              Consent of Arthur Andersen LLP

23.2              Consent of Seth H. Hoogasian, Esq.
                  (contained in his opinion filed as
                  Exhibit 5.1).

24                Power of Attorney (see  signature  pages to this  Registration
                  Statement).
EXHIBIT 5.1

                                 Thermedics Inc.
                               470 Wildwood Street
                        Woburn, Massachusetts 01888-1799

                                                  April 30, 1999

Thermedics Inc.
470 Wildwood Street
Woburn, Massachusetts 01888-1799

Re:   Registration Statement on Form S-8
      Relating to 1,294,350 Shares of the Common Stock,
      $.10 par value, of Thermedics Inc.

Dear Sirs:

     I am General Counsel to Thermedics  Inc., a Massachusetts  corporation (the
"Company"),  and have acted as counsel in connection with the registration under
the  Securities  Act of  1933,  as  amended,  on  Form  S-8  (the  "Registration
Statement"),  of 1,294,350 shares of the Company's Common Stock,  $.10 par value
per share (the "Shares") subject to the following  employee benefit plans (each,
a "Plan"):  Thermedics Inc.  Employees  Equity  Incentive Plan;  Thermedics Inc.
Deferred Compensation Plan for Directors;  Thermo Voltek Corp.  Nonqualified and
Incentive Stock Option Plan;  Thermo Voltek Corp.  Equity Incentive Plan; Thermo
Voltek Corp.  Directors Stock Option Plan;  Thermedics  Inc./Thermo Voltek Corp.
Nonqualified  Stock Option Plan;  Thermedics  Inc.  Directors Stock Option Plan;
Thermo Electron  Corporation/Thermo Voltek Corp. Nonqualified Stock Option Plan;
Thermo  Electron  Corporation  Directors  Stock Option Plan; and Thermo Electron
Corporation/Thermedics Inc. Nonqualified Stock Option Plan.

      I or a member of my legal staff have  reviewed the  corporate  proceedings
taken by the Company  with respect to the  authorization  of the issuance of the
Shares.  I or a member of my legal  staff have also  examined  and  relied  upon
originals or copies, certified or otherwise authenticated to my satisfaction, of
all corporate records, documents, agreements or other instruments of the Company
and have made all  investigations  of law and have  discussed with the Company's
representatives   all  questions  of  fact  that  I  have  deemed  necessary  or
appropriate.
Based upon and subject to the foregoing, I am of the opinion that:

      1. The Company is a corporation  duly organized,  validly  existing and in
corporate good standing under the laws of the Commonwealth of Massachusetts.

      2. The issuance and sale of the Shares as contemplated in the Registration
Statement have been duly authorized by the Company.

      3. The Shares,  when issued and sold in accordance  with the provisions of
the applicable Plan, will be validly issued, fully paid and nonassessable.

      I hereby  consent  to the filing of this  opinion  as  Exhibit  5.1 to the
Registration Statement.

                                          Very truly yours,

                                          /s/ Seth H. Hoogasian
                                          Seth H. Hoogasian
                                          General Counsel
Exhibit 23.1

                    Consent of Independent Public Accountants
                    -----------------------------------------

     As independent public  accountants,  we hereby consent to the incorporation
by  reference  in this  Registration  Statement  on Form S-8 of our report dated
February 11, 1999,  included or incorporated  by reference in Thermedics  Inc.'s
Annual  Report on Form  10-K for the year  ended  January  2,  1999,  and to all
references to our Firm included in this Registration Statement.

                                                  ARTHUR ANDERSEN LLP

Boston, Massachusetts
April 28, 1999