Exhibit 4.3


                          FIRST SUPPLEMENTAL INDENTURE


             FIRST  SUPPLEMENTAL  INDENTURE  (this  "First   Supplemental
        Indenture") dated as of February 6, 1998, by and among ThermoTrex
        Corporation,  a  Delaware  corporation  (the  "Issuer"),   Thermo
        Electron Corporation,  a Delaware  corporation (the  "Guarantor")
        and Bankers Trust  Company, a New  York banking corporation  (the
        "Trustee").

             WHEREAS, the  Issuer, the  Guarantor  and the  Trustee  have
        entered into that certain Indenture dated as of October 28,  1997
        (the "Indenture"), with respect to  the issuance and sale of  the
        Issuer's subordinated debt securities (the "Securities"); and

             WHEREAS, Article  Twelve of  the Indenture  ("Conversion  of
        Securities") contains provisions  permitting the  Issuer, in  its
        sole discretion, to (i) pay cash  in respect of all or a  portion
        of the shares of the common  stock, $.01 par value per share,  of
        the Issuer  (the  "Common  Stock") otherwise  issuable  upon  the
        conversion of  the  Securities  (the  "Cash  Settlement  Option")
        and/or (ii) deliver fully paid  and non-assessable shares of  the
        common stock, $1.00 par value per share, of the Guarantor in lieu
        of issuing  shares of  Common Stock  upon the  conversion of  the
        Securities (the "Stock Settlement Option"); and

             WHEREAS, the Board of Directors of the Issuer has determined
        that the Cash Settlement Option  and the Stock Settlement  Option
        do not serve any desired purpose of the Issuer; and
         
             WHEREAS, the  Issuer and  the  Guarantor have  the  ability,
        pursuant to Section 901 of the Indenture, to amend the  Indenture
        to  remove  both  the  Cash  Settlement  Option  and  the   Stock
        Settlement Option, upon  the authorization of  such amendment  by
        the Boards of Directors  of the Issuer  and the Guarantor,  which
        authorization has been received; and

             WHEREAS, as required  by Section 903  of the Indenture,  the
        Issuer and the Guarantor  have provided the  Trustee with (i)  an
        Opinion of Counsel of the  Issuer and the Guarantor stating  that
        the execution of this First Supplemental Indenture is  authorized
        by the Indenture  and (ii) Officers'  Certificates of the  Issuer
        and the Guarantor  stating that all  conditions precedent to  the
        execution  of  this  First   Supplemental  Indenture  have   been
        fulfilled;

             NOW, THEREFORE, in consideration of the foregoing and  other
        good and valuable consideration,  the receipt and sufficiency  of
        which are  hereby  acknowledged,  the  parties  hereto  agree  as
        follows:

             1.   The Issuer shall no longer have the ability to exercise
        either the Cash Settlement Option or the Stock Settlement  Option
        with respect to any Securities issued pursuant to the Indenture.
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             2.   This First Supplemental Indenture shall form a part  of
        the Indenture for all  purposes, and every  Holder of a  Security
        authenticated and delivered  pursuant to  the Indenture  (whether
        before or after the date hereof) shall be bound hereby.

             3.   Except as amended by this First Supplemental Indenture,
        the Indenture shall  continue in  full force and  effect and  the
        Indenture is in all respects hereby ratified and confirmed.

             4.   In  case  any  provision  of  this  First  Supplemental
        Indenture  shall  be  invalid,  illegal  or  unenforceable,   the
        validity, legality and enforceability of the remaining provisions
        hereof and of the Indenture shall  not be in any way affected  or
        impaired.

             5.   This First Supplemental  Indenture may  be executed  in
        any number of counterparts,  each of which  so executed shall  be
        deemed to  be  an  original,  but  all  such  counterparts  shall
        together constitute but one and the same instrument.

             6.   All capitalized terms defined in the Indenture and used
        herein without definition shall have the same meanings herein  as
        are ascribed to them in the Indenture.

             IN WITNESS  WHEREOF, the  parties  hereto have  caused  this
        First Supplemental Indenture to be  duly executed as of the  date
        first above written.

                                           THERMOTREX CORPORATION


                                           ------------------------------
                                           By:  Melissa F. Riordan
                                           Its:  Treasurer


                                           THERMO ELECTRON CORPORATION


                                           ------------------------------
                                           By:  John N. Hatsopoulos
                                           Its:  President and Chief
                                                 Financial Officer


                                           BANKERS TRUST COMPANY

                                           ------------------------------
                                           By: Sandra J. Shaffer
                                           Its: Assistant Vice President