1
                                                                 EXHIBIT (a)(20)

                              LETTER OF TRANSMITTAL
                     TO EXCHANGE 0.45 SHARES OF COMMON STOCK

                                       of
                           THERMO ELECTRON CORPORATION

                                       for
                     EACH OUTSTANDING SHARE OF COMMON STOCK

                                       of
                                 THERMEDICS INC.
                      and cash in lieu of fractional shares
        Pursuant to the Merger with and into Thermo Electron Corporation
                             Effective June 30, 2000

- --------------------------------------------------------------------------------
         BOX A TO BE COMPLETED BY EACH HOLDER OF THERMEDICS COMMON STOCK



- ------------------------------------------------------------------------- ------------------------ ------------------------------
            NAME(S) AND ADDRESS(ES) OF REGISTERED HOLDER(S)                     CERTIFICATE              NUMBER OF SHARES
                    (AS IT APPEARS ON CERTIFICATES)*                            NUMBER(S)**         OF THERMEDICS COMMON STOCK
- ------------------------------------------------------------------------- ------------------------ ------------------------------
                                                                                           

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

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

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

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

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

- ------------------------------------------------------------------------- ------------------------ ------------------------------
- ------------------------------------------------------------------------- ------------------------ ------------------------------
                                                                          Total Number of Shares
                                                                           of Thermedics Common
                                                                                   Stock
- ------------------------------------------------------------------------- ------------------------ ------------------------------


*Attach Schedule if needed.  See Instruction 3.

|_|  I HAVE LOST MY THERMEDICS CERTIFICATE(S) FOR ______ SHARES OF THERMEDICS
     COMMON STOCK, AND REQUIRE ASSISTANCE WITH RESPECT TO REPLACING SUCH
     CERTIFICATE(S). SEE INSTRUCTION 8.


     THIS LETTER OF TRANSMITTAL, INCLUDING THE SUBSTITUTE FORM W-9 ON PAGE 7 (OR
FOR NON-U.S. PERSONS, A SUBSTITUTE FORM W-8), SHOULD BE COMPLETED, SIGNED AND
SUBMITTED, TOGETHER WITH YOUR CERTIFICATE(S) EVIDENCING SHARES OF COMMON STOCK
OF THERMEDICS INC. TO:


                                                                              

        By First Class Mail:                              By Hand:                        By Overnight, Certified or Express Mail:
    EquiServe Trust Company, N.A.      Securities Transfer & Reporting Services, Inc.           EquiServe Trust Company, N.A.
            P.O. Box 8029                      EquiServe Trust Company, N.A.                150 Royall Street, Mail Stop 45-01-40
  Boston, Massachusetts 02266-8029              100 William Street, Galleria                     Canton, Massachusetts 02021
   Attn: Corporate Reorganizations                New York, New York 10038                     Attn: Corporate Reorganizations



     For additional information please contact EquiServe at (781) 575-3120.


                                       1



   2


Ladies and Gentlemen:

     The undersigned has been advised that on June 30, 2000 (the "Effective
Date"), Thermedics Inc. ("Thermedics" or the "Company") was merged with and into
Thermo Electron Corporation (the "Merger"). Pursuant to the Merger, each share
of the Company's common stock, $.10 par value per share ("Thermedics Common
Stock"), outstanding at the effective time of the Merger was converted into the
right to receive 0.45 shares of common stock, $1.00 par value per share, of
Thermo Electron Corporation ("Thermo Electron Common Stock") and cash in lieu of
fractional shares of Thermo Electron Common Stock (the "Merger Consideration").
The undersigned herewith surrenders the certificate(s) listed below (the
"Certificates"), which represent shares of Thermedics Common Stock.

              PLEASE CAREFULLY READ THE ACCOMPANYING INSTRUCTIONS.

     Unless otherwise requested in the special instructions in Boxes B or C
below, the undersigned requests that the certificate(s) for Thermo Electron
Common Stock and any check in lieu of fractional shares issued pursuant to the
Merger be issued in the name(s) and mailed to the address(es) set forth on the
preceding page.

     The undersigned hereby irrevocably constitutes and appoints EquiServe Trust
Company, N.A., as exchange agent (the "Exchange Agent"), the true and lawful
attorney-in-fact of the undersigned with respect to the Certificates with full
power of substitution (such power of attorney being deemed to be an irrevocable
power coupled with an interest) to deliver such Certificates on the account
books maintained by the Exchange Agent and to deliver as the undersigned's agent
the Merger Consideration to which the undersigned is entitled upon surrender of
the Certificates.

     All authority herein conferred or agreed to be conferred shall survive the
death or incapacity of the undersigned, and any obligation of the undersigned
hereunder shall be binding upon the heirs, personal representatives, estates,
successors and assigns of the undersigned. The undersigned hereby represents and
warrants that the undersigned has full power and authority to submit the
Certificates submitted hereby, free and clear of all liens and encumbrances and
not subject to any adverse claim, unless otherwise noted hereon. The undersigned
will, upon request, execute and deliver any additional documents necessary or
desirable to complete the exchange of such Certificates for the Merger
Consideration.



- -----------------------------------------------------------     ----------------------------------------------------------
BOX B            SPECIAL ISSUANCE                               BOX C                SPECIAL DELIVERY
                   INSTRUCTIONS                                                        INSTRUCTIONS
    (SEE INSTRUCTIONS 1, 4, 5 AND 6 ON PAGES 4 AND 5.)               (SEE INSTRUCTIONS 1, 4 AND 6 ON PAGES 4 AND 5.)
                                                         

To be completed ONLY if the certificate(s) for                        To be completed ONLY if the certificate(s) for
Thermo Electron Common Stock and any check in lieu                    Thermo Electron Common Stock and any check in lieu of
of fractional shares should be issued in the name of                  fractional shares should be sent to someone other than
someone other than other than as set forth in Box A.                  as set forth in Box A or to an address other than as
                                                                      set forth in Box A.

ISSUE         CHECK           CERTIFICATE(S) TO:
                                                                MAIL      CHECK             CERTIFICATE(S) TO:
Name: _______________________________________
                  (Please Print)
                                                                Name: ______________________________________
Address: ____________________________________                                     (Please Print)

_____________________________________________                    Address: ___________________________________

_____________________________________________                    ____________________________________________
              (Including Zip Code)
                                                                 ____________________________________________
                                                                              (Including Zip Code)
- --------------------------------------------
    (Tax ID or Social Security Number)
 (See accompanying Substitute Form W-9.)
- -----------------------------------------------------------     ----------------------------------------------------------


                                       2



   3


- --------------------------------------------------------------------------------
BOX D
                                 SIGN HERE (SEE
                     INSTRUCTIONS 1 AND 4 ON PAGES 4 AND 5.)
             (COMPLETE ACCOMPANYING SUBSTITUTE FORM W-9 ON PAGE 7.)

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

- --------------------------------------------------------------------------------
                         Signature(s) of Shareholder(s)

Date:                            , 2000
      ---------------------------


     (MUST BE SIGNED BY THE REGISTERED HOLDER(S) EXACTLY AS NAME(S) APPEAR(S) ON
CERTIFICATE(S) OR BY PERSONS AUTHORIZED TO BECOME REGISTERED HOLDER(S) BY
CERTIFICATES AND DOCUMENTS TRANSMITTED HEREWITH. IF SIGNATURE IS BY TRUSTEES,
EXECUTORS, ADMINISTRATORS, GUARDIANS, ATTORNEYS-IN-FACT, OFFICERS OR
CORPORATIONS OR OTHERS ACTING IN A FIDUCIARY OR REPRESENTATIVE CAPACITY, PLEASE
PROVIDE THE FOLLOWING INFORMATION. SEE INSTRUCTION 4.)

Name(s):  ----------------------------------------------------------------------

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

- --------------------------------------------------------------------------------
                                 (Please Print)


Capacity (Full Title):
                        --------------------------------------------------------
Address:
         -----------------------------------------------------------------------

- --------------------------------------------------------------------------------
                               (Including Zip Code)


Area Code and Telephone Number: (      )
                                 ------ ----------------------------------------

Tax Identification or Social Security Number:
                                             -----------------------------------

Date:                    , 2000
      -------------------
- --------------------------------------------------------------------------------

PLACE SIGNATURE MEDALLION GUARANTEE ON THIS PAGE IF REQUIRED. (SEE INSTRUCTIONS
1 AND 4 ON PAGES 4 AND 5.)



                                       3

   4


                                  INSTRUCTIONS

     A shareholder of the Company will not receive the Merger Consideration in
exchange for such holder's Certificates until the Certificates owned by such
holder are received by the Exchange Agent at one of the addresses set forth on
page 1, together with such documents as the Exchange Agent may require, and
until the same are processed by the Exchange Agent. No interest will accrue on
any amounts due.

     1. GUARANTEE OF SIGNATURES. No signature guarantee on this Letter of
Transmittal is required (a) if this Letter of Transmittal is signed by the
registered holder(s) of the Certificate(s) surrendered herewith, unless such
holder has (have) completed either Box B entitled "Special Issuance
Instructions" or Box C entitled "Special Delivery Instructions" on this Letter
of Transmittal, or (b) if such Certificates are surrendered for the account of a
firm that is a commercial bank, broker, dealer, credit union, savings
association or other entity that is a member in good standing of the Securities
Transfer Agents Medallion Program, the Stock Exchanges' Medallion Program or the
New York Stock Exchange, Inc. Medallion Signature Program (each of the foregoing
being referred to as an "Eligible Institution"). In all other cases, all
signatures on this Letter of Transmittal must be guaranteed by an Eligible
Institution. See Instruction 4.

     2. DELIVERY OF LETTER OF TRANSMITTAL AND CERTIFICATES. Please do not send
the Certificate(s) directly to Thermo Electron Corporation. The Certificate(s),
together with a properly completed and duly executed copy of this Letter of
Transmittal or a photocopy hereof, and any other documents required by this
Letter of Transmittal, should be delivered to the Exchange Agent at one of the
addresses set forth on page 1 of this Letter of Transmittal.

     THE METHOD OF DELIVERY OF ALL DOCUMENTS IS AT THE OPTION AND RISK OF THE
OWNER. HOWEVER, IF CERTIFICATE(S) ARE SENT BY MAIL, IT IS RECOMMENDED THAT THEY
BE SENT BY REGISTERED MAIL, PROPERLY INSURED, WITH RETURN RECEIPT REQUESTED.
RISK OF LOSS AND TITLE TO THE CERTIFICATE(S) SHALL PASS UPON DELIVERY OF THE
CERTIFICATE(S) TO THE EXCHANGE AGENT.

     All questions as to validity, form and eligibility of any surrender of any
Certificate hereunder will be determined by Thermo Electron Corporation (which
may delegate power in whole or in part to the Exchange Agent) and such
determination shall be final and binding. Thermo Electron Corporation reserves
the right to waive any irregularities or defects in the surrender of any
Certificate(s). A surrender will not be deemed to have been made until all
irregularities have been cured or waived.

     3. INADEQUATE SPACE. If the space provided in this Letter of Transmittal is
inadequate, the Certificate number(s) and the number of shares of Thermedics
Common Stock represented thereby should be listed on a separate signed schedule
attached hereto and referenced in Box A.

     4. SIGNATURES ON LETTER OF TRANSMITTAL, STOCK POWERS AND ENDORSEMENTS. If
the registered holder(s) of the Certificate(s) surrendered hereby signs this
Letter of Transmittal, the signature must correspond with the name that is
written on the face of the Certificate(s) without alteration, enlargement or any
change whatsoever.

     If the Certificate(s) surrendered are owned of record by two or more joint
owners, all such owners must sign this Letter of Transmittal.

     If any shares of Thermedics Common Stock are registered in different names
on several Certificates, it will be necessary to complete, sign and submit as
many separate Letters of Transmittal as there are different registrations of
Certificates.

     When this Letter of Transmittal is signed by the registered owner(s) of the
Certificate(s) listed and surrendered herewith, no endorsements of the
Certificate(s) or separate stock powers are required.


                                       4

   5


     If this Letter of Transmittal is signed by a person other than the
registered owner of the Certificate(s) listed, such Certificate(s) must be
endorsed or accompanied by appropriate stock power(s), in either case signed by
the registered owner or owners or a person with full authority to sign on behalf
of the registered owner(s). Signatures on such Certificate(s) or stock power(s)
must be guaranteed by an Eligible Institution. See Instruction 1.

     If this Letter of Transmittal or any Certificate(s) or stock power(s) is
signed by an executor, administrator, trustee, guardian, attorney, officer of a
corporation or others acting in a fiduciary or representative capacity, such
persons must so indicate when signing, must give his or her full title in such
capacity, and evidence satisfactory to the Exchange Agent of his or her
authority to so act must be submitted. The Exchange Agent will not exchange any
Certificate(s) until all instructions herein are complied with or waived by
Thermo Electron Corporation.

     5. STOCK TRANSFER TAXES. In the event that any transfer or other taxes
become payable by reason of the issuance of the certificate(s) for Thermo
Electron Common Stock and any check in lieu of fractional shares in any name
other than that of the record holder, such transferee or assignee must pay such
tax to the Exchange Agent or must establish to the satisfaction of the Exchange
Agent that such tax has been paid or is not applicable prior to the delivery of
the Merger Consideration.

     6. SPECIAL ISSUANCE AND DELIVERY INSTRUCTIONS. Indicate on the "Special
Issuance Instructions" or the "Special Delivery Instructions" box the name and
address of the person(s) to whom the certificate(s) for Thermo Electron Common
Stock and any check in lieu of fractional shares is to be sent if different from
the name or address of the person(s) signing this Letter of Transmittal.

     7. SUBSTITUTE FORM W-9. Under U.S. federal income tax law, each shareholder
surrendering Certificates is required to supply the Exchange Agent with such
holder's correct Taxpayer Identification Number ("TIN") (e.g., social security
number or employer identification number) on the Substitute Form W-9 on page 7,
and to certify whether such holder is subject to backup withholding of federal
income tax. If the holder is subject to backup withholding, then such holder
must cross out item (2) in Part III of the Substitute Form W-9. Failure to
provide such information on the form may subject the holder to federal income
tax withholding at a rate of 31% on the payment of any cash in lieu of
fractional shares. If the Exchange Agent is not provided with the correct TIN,
the holder may be subject to a penalty imposed by the Internal Revenue Service
(the "IRS").

     Certain holders of Certificate(s) (including, among others, all
corporations and certain foreign individuals) are not subject to these backup
withholding and reporting requirements. Exempt holders of Certificate(s) should
indicate their exempt status on Substitute Form W-9 by writing the word "EXEMPT"
on the line for the Social Security Number in Part I. A foreign individual may
qualify as an exempt recipient by submitting to the Exchange Agent a properly
completed IRS Substitute Form W-8 (that the Exchange Agent will supply upon
request), signed under penalties of perjury, attesting to their foreign status.
See the enclosed "Guidelines For Certification of Taxpayer Identification Number
on Substitute Form W-9" for additional instructions.

     IF (I) THE HOLDER DOES NOT FURNISH THE EXCHANGE AGENT WITH A TIN IN THE
REQUIRED MANNER, (II) THE IRS NOTIFIES THE EXCHANGE AGENT THAT THE TIN PROVIDED
IS INCORRECT, OR (III) THE HOLDER IS REQUIRED BUT FAILS TO CERTIFY THAT IT IS
NOT SUBJECT TO BACKUP WITHHOLDING, BACKUP WITHHOLDING WILL APPLY. IF BACKUP
WITHHOLDING APPLIES, THE EXCHANGE AGENT IS REQUIRED TO WITHHOLD 31% OF ANY CASH
PAYMENTS MADE TO THE HOLDER OR OTHER PAYEE. BACKUP WITHHOLDING IS NOT AN
ADDITIONAL TAX. RATHER, THE TAX LIABILITY OF PERSONS SUBJECT TO BACKUP
WITHHOLDING WILL BE REDUCED BY THE AMOUNT OF TAX WITHHELD. IF WITHHOLDING
RESULTS IN AN OVERPAYMENT OF TAXES, A REFUND MAY BE OBTAINED FROM THE IRS.

     If the surrendering holder has not been issued a TIN and has applied for a
TIN or intends to apply for a TIN in the near future, then the box in Part II of
the form should be checked. If the box in Part II is checked, the surrendering
holder must also complete the "Certificate of Awaiting Taxpayer Identification
Number" at the

                                       5

   6


bottom of page 7 in order to avoid backup withholding. If you have checked the
box in Part II and do not provide the Exchange Agent with a properly certified
TIN within 60 days, the Exchange Agent will withhold 31% of all reportable cash
payments made.

     The stockholder is required to give the Exchange Agent the TIN of the
record holder of the Certificate(s) or of the last transferee appearing on the
transfers attached to, or endorsed on, the Certificate(s). If the Certificate(s)
are in more than one name or are not in the name of the actual owner, consult
the enclosed "Guidelines For Certification of Taxpayer Identification Number on
Substitute Form W-9" for additional guidance on which number to report.

     8. LOST, STOLEN OR DESTROYED CERTIFICATES. In the event that any Thermedics
stockholder is unable to deliver to the Exchange Agent the Certificate(s)
representing his, her or its shares of Thermedics Common Stock due to the loss
or destruction of such Certificate(s), such fact should be indicated on the face
of this Letter of Transmittal. In such case, the stockholder should also contact
the Exchange Agent, at (781) 575-3120, to report the lost securities. The
Exchange Agent will forward additional documentation which such stockholder must
complete in order to effectively surrender such lost or destroyed Certificate(s)
(including affidavits of loss and indemnity bonds in lieu thereof). There may be
a fee in respect of lost or destroyed Certificates, but surrenders hereunder
regarding such lost Certificate(s) will be processed only after such
documentation has been submitted to and approved by the Exchange Agent.

     9. INFORMATION AND ADDITIONAL COPIES. Information and additional copies of
this Letter of Transmittal may be obtained from the Exchange Agent by writing to
one of the addresses or calling the number listed on page 1 of this Letter of
Transmittal.

                                       6

   7


                TO BE COMPLETED BY ALL HOLDERS OF THERMEDICS INC.
                                  COMMON STOCK

                   PAYER'S NAME: EQUISERVE TRUST COMPANY, N.A.

                                                                           

- --------------------------------- ------------------------------------------------ -------------------------------------------------
SUBSTITUTE                        PART I - PLEASE  PROVIDE YOUR TIN IN THE BOX AT
                                  RIGHT AND CERTIFY BY SIGNING AND DATING BELOW.             _______________________________
FORM W-9                                                                                         Social Security Number
                                                                                                           or
DEPARTMENT OF THE TREASURY
INTERNAL REVENUE SERVICE                                                                     _______________________________
                                                                                             Employer Identification Number
PAYER'S REQUEST FOR TAXPAYER
IDENTIFICATION NUMBER ("TIN")

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

                                  Name (Please Print) ______________________________________________________________________

                                  Address _______________________________________________________________________________

                                  City ____________________________________________ State _____________ ZIP Code ____________

- --------------------------------- --------------------------------------------------------------------------------------------------
- --------------------------------- --------------------------------------------------------------------------------------------------
                                  PART II - Awaiting TIN

- --------------------------------- --------------------------------------------------------------------------------------------------
- --------------------------------- --------------------------------------------------------------------------------------------------
                                  PART III - CERTIFICATION - Under penalties of perjury, I certify that:

                                  (1)      The number shown on this form is my correct TIN (or a TIN has not been
                                           issued to me but I have mailed or delivered an application to receive a
                                           TIN or intend to do so in the near future);

                                  (2)      I am not subject to backup withholding because: (a) I am exempt
                                           from backup withholding, or (b) I have not been notified by the
                                           Internal Revenue Service ("IRS") that I am subject to backup withholding as
                                           a result of a failure to report all interest or dividends, or (c) the IRS
                                           has notified me that I am no longer subject to backup withholding; and

                                  (3)      All other information provided on this form is true, correct and complete.

                                           You must cross out item (2) above if you have been notified by the IRS that you
                                           are subject to backup withholding because of underreporting interest or
                                           dividends on your tax return and you have not been advised by the IRS that such
                                           backup withholding has been terminated.
- ------------------------------------------------------------------------------------------------------------------------------------



           FAILURE TO COMPLETE AND RETURN THIS SUBSTITUTE FORM W-9 MAY
                         RESULT IN BACKUP WITHHOLDING.

- --------------------------------------------------------------------------------
Sign
Here:   Signature: ______________________________ Date: __________________, 2000
- --------------------------------------------------------------------------------


      YOU MUST COMPLETE THE FOLLOWING CERTIFICATE IF YOU CHECKED THE BOX IN
                      PART II OF THE SUBSTITUTE FORM W-9.

- --------------------------------------------------------------------------------
             CERTIFICATE OF AWAITING TAXPAYER IDENTIFICATION NUMBER

     I certify under the penalties of perjury that a TIN has not been issued to
me, and either (1) I have mailed or delivered an application to receive a TIN to
the appropriate IRS Center or Social Security Administration Office or (2) I
intend to mail or deliver an application in the near future. I understand that
if I do not provide a TIN within 60 days, 31% of all reportable cash payments
made to me may be withheld until I provide a TIN.

Signature:_____________________________________  Date: __________________, 2000


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



                                       7