EXHIBIT 10.1



        THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER
        THE SECURITIES ACT OF 1933, AS AMENDED (THE "ACT").  THESE
        SECURITIES HAVE BEEN ACQUIRED FOR INVESTMENT, AND NOT WITH A VIEW
        TO DISTRIBUTION OR RESALE, AND MAY NOT BE SOLD, PLEDGED,
        MORTGAGED, HYPOTHECATED OR OTHERWISE TRANSFERRED (1) WITHOUT AN
        EFFECTIVE REGISTRATION STATEMENT UNDER THE ACT COVERING THESE
        SECURITIES OR (2) UNLESS AN EXEMPTION FROM REGISTRATION IS
        AVAILABLE.



                             THERMOTREX CORPORATION
                       Promissory Note Due October 5, 1998
                             Waltham, Massachusetts

                                                            July 31, 1997


             For value received, ThermoTrex Corporation, a Delaware
        corporation (the "Company"), hereby promises to pay to Thermo
        Electron Corporation (hereinafter referred to as the "Payee"), or
        registered assigns, on a subordinated basis, on October 5, 1998,
        as described below, the principal sum of eleven million dollars
        ($11,000,000) or such part thereof as then remains unpaid, to pay
        interest from the date hereof on the whole amount of said
        principal sum remaining from time to time unpaid at a rate per
        annum equal to the rate of the Commercial Paper Composite Rate as
        reported by Merrill Lynch Capital Markets, as an average of the
        last five business days of the fiscal quarter, plus twenty-five
        (25) basis points, such interest to be payable in arrears on the
        first day of each fiscal quarter of the Company during the term
        set forth herein, until the whole amount of the principal hereof
        remaining unpaid shall become due and payable, and to pay
        interest on all overdue principal and interest at a rate per
        annum equal to the rate of interest announced from time to time
        by The First National Bank of Boston at its head office in
        Boston, Massachusetts as its "base rate" plus one percent (1%).
        Principal and all accrued but unpaid interest shall be repaid on
        October 5, 1998.  Principal and interest shall be payable in
        lawful money of the United States of America, in immediately
        available funds, at the principal office of the Payee or at such
        other place as the legal holder may designate from time to time
        in writing to the Company.  Interest shall be computed on an
        actual 360-day basis.

             This Note may be prepaid at any time or from time to time,
        in whole or in part, without any premium or penalty.  All
        prepayments shall be applied first to accrued interest and then
        to principal.
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             The then unpaid principal amount of, and interest
        outstanding on, this Note shall be and become immediately due and
        payable without notice or demand, at the option of the holder
        hereof, upon the occurrence of any of the following events:

                  (a)  the failure of the Company to pay any amount due
             hereunder within ten (10) days of the date when due;

                  (b)  any representation, warranty or statement made or
             furnished to the Payee by the Company in connection with
             this Note or the transaction from which it arises shall
             prove to have been false or misleading in any material
             respect as of the date when made or furnished;

                  (c)  the failure of the Company to pay its debts as
             they become due, the insolvency of the Company, the filing
             by or against the Company of any petition under the U.S.
             Bankruptcy Code (or the filing of any similar petition under
             the insolvency law of any jurisdiction), or the making by
             the Company of an assignment or trust mortgage for the
             benefit of creditors or the appointment of a receiver,
             custodian or similar agent with respect to, or the taking by
             any such person of possession of, any property of the
             Company;

                  (d)  the sale by the Company of all or substantially
             all of its assets;

                  (e)  the merger or consolidation of the Company with or
             into any other corporation in a transaction in which the
             Company is not the surviving entity;

                  (f)  the issuance of any writ of attachment, by trustee
             process or otherwise, or any restraining order or injunction
             not removed, repealed or dismissed within thirty (30) days
             of issuance, against or affecting the person or property of
             the Company or any liability or obligation of the Company to
             the holder hereof; and

                  (g)  the suspension of the transaction of the usual
             business of the Company.

             Upon surrender of this Note for transfer or exchange, a new
        Note or new Notes of the same tenor dated the date to which
        interest has been paid on the surrendered Note and in an
        aggregate principal amount equal to the unpaid principal amount
        of the Note so surrendered will be issued to, and registered in
        the name of, the transferee or transferees.  The Company may
        treat the person in whose name this Note is registered as the
        owner hereof for the purpose of receiving payment and for all
        other purposes.
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             In case any payment herein provided for shall not be paid
        when due, the Company further promises to pay all cost of
        collection, including all reasonable attorneys' fees.

             No delay or omission on the part of the Payee in exercising
        any right hereunder shall operate as a waiver of such right or of
        any other right of the Payee, nor shall any delay, omission or
        waiver on any one occasion be deemed a bar to or waiver of the
        same or any other right on any future occasion.  The Company  
        hereby waives presentment, demand, notice of prepayment, protest
        and all other demands and notices in connection with the
        delivery, acceptance, performance, default or enforcement of this
        Note.  The undersigned hereby assents to any indulgence and any
        extension of time for payment of any indebtedness evidenced
        hereby granted or permitted by the Payee.  

             This Note shall be governed by and construed in accordance
        with, the laws of the Commonwealth of Massachusetts and shall
        have the effect of a sealed instrument.


                                      THERMOTREX CORPORATION



                                        By: 
                                           __________________________________
                                           Gary S. Weinstein
                                           Chairman and Chief Executive Officer

        [Corporate Seal]

        Attest:



        ____________________________
        Sandra L. Lambert
        Secretary



        cc:  Seth Hoogasian
             Maureen Jacobs
             Sandra Lambert
             Karen Levin
             Andy Pilla
             Gina Silvestri
             David Teitel
             Chris Vinchesi



AA972120022           
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