Exhibit 10.6
              AMENDED AND RESTATED MASTER GUARANTEE REIMBURSEMENT
                               AND LOAN AGREEMENT


             This AGREEMENT is entered into as of the 5th day of
        December, 1997, by and among Thermo Instrument Systems Inc. (the
        "Parent") and those of its subsidiaries that join in this
        Agreement by executing the signature page hereto (the "Majority
        Owned Subsidiaries").

                                   WITNESSETH:

             WHEREAS, the Majority Owned Subsidiaries and their
        wholly-owned subsidiaries wish to enter into various financial
        transactions, such as convertible or nonconvertible debt, loans,
        and equity offerings, and other contractual arrangements with
        third parties (the "Underlying Obligations") and may provide
        credit support to, on behalf of or for the benefit of, other
        subsidiaries of the Parent ("Credit Support Obligations"); 

             WHEREAS, the Majority Owned Subsidiaries and the Parent
        acknowledge that the Majority Owned Subsidiaries and their
        wholly-owned subsidiaries may be unable to enter into many kinds
        of Underlying Obligations without a guarantee of their
        performance thereunder from the Parent (a "Parent Guarantee") or
        without obtaining Credit Support Obligations from other Majority
        Owned Subsidiaries;

             WHEREAS, the Majority Owned Subsidiaries and their
        wholly-owned subsidiaries may borrow funds from the Parent, and
        the Parent may loan funds or provide credit to the Majority Owned
        Subsidiaries and their wholly-owned subsidiaries, on a short-term
        and unsecured basis; and

             WHEREAS, the Parent is willing to consider continuing to
        issue Parent Guarantees and providing credit, and the Majority
        Owned Subsidiaries are willing to consider continuing to provide
        Credit Support Obligations and to borrow funds, on the terms and
        conditions set forth below;

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

        1.   If the Parent provides a Parent Guarantee of an Underlying
             Obligation, and the beneficiary(ies) of the Parent Guarantee
             enforce the Parent Guarantee, or the Parent performs under
             the Parent Guarantee for any other reason, then the Majority
             Owned Subsidiary that is obligated, either directly or
             indirectly through a wholly-owned subsidiary, under such
             Underlying Obligation shall indemnify and save harmless the
             Parent from any liability, cost, expense or damage
             (including reasonable attorneys' fees) suffered by the
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             Parent as a result of the Parent Guarantee.  If a Majority
             Owned Subsidiary or a wholly-owned subsidiary thereof
             provides a Credit Support Obligation for any subsidiary of
             the Parent, other than a subsidiary of such Majority Owned
             Subsidiary, and the 
             beneficiary(ies) of the Credit Support Obligation enforce
             the Credit Support Obligation, or the Majority Owned
             Subsidiary or its wholly-owned subsidiary  performs under
             the Credit Support Obligation for any other reason, then the
             Parent shall indemnify and save harmless the Majority Owned
             Subsidiary or its wholly-owned subsidiary, as applicable,
             from any liability, cost, expense or damage (including
             reasonable attorneys' fees) suffered by the Majority Owned
             Subsidiary or its wholly-owned subsidiary, as applicable, as
             a result of the Credit Support Obligation.  Without limiting
             the foregoing, Credit Support Obligations include the
             deposit of funds by a Majority Owned Subsidiary or a
             wholly-owned subsidiary thereof in a credit arrangement with
             a banking facility whereby such funds are available to the
             banking facility as collateral for overdraft obligations of
             other Majority Owned Subsidiaries or their subsidiaries also
             participating in the credit arrangement with such banking
             facility.

        2.   For purposes of this Agreement, the term "guarantee" shall
             include not only a formal guarantee of an obligation, but
             also any other arrangement where the Parent is liable for
             the obligations of a Majority Owned Subsidiary or its
             wholly-owned subsidiaries.  Such other arrangements include
             (a) representations, warranties and/or covenants or other
             obligations joined in by the Parent, whether on a joint or
             joint and several basis, for the benefit of the Majority
             Owned Subsidiary or its wholly-owned subsidiaries and (b)
             responsibility of the Parent by operation of law for the
             acts and omissions of the Majority Owned Subsidiary or its
             wholly-owned subsidiaries, including controlling person
             liability under securities and other laws.

        3.   Promptly after the Parent receives notice that a beneficiary
             of a Parent Guarantee is seeking to enforce such Parent
             Guarantee, the Parent shall notify the Majority Owned
             Subsidiary(s) obligated, either directly or indirectly
             through a wholly-owned subsidiary, under the relevant
             Underlying Obligation.  Such Majority Owned Subsidiary(s) or
             wholly-owned subsidiary thereof, as applicable, shall have
             the right, at its own expense, to contest the claim of such
             beneficiary.  If a Majority Owned Subsidiary or wholly-owned
             subsidiary thereof, as applicable, is contesting the claim
             of such beneficiary, the Parent will not perform under the
             relevant Parent Guarantee unless and until, in the Parent's
             reasonable judgment, the Parent is obligated under the terms
             of such Parent Guarantee to perform.  Subject to the
             foregoing, any dispute between a Majority Owned Subsidiary
             or wholly-owned subsidiary thereof, as applicable, and a
             beneficiary of a Parent Guarantee shall not affect such
             Majority Owned Subsidiary's obligation to promptly indemnify
             the Parent hereunder.  Promptly after a Majority Owned
             Subsidiary or wholly-owned subsidiary thereof, as
             applicable, receives notice that a beneficiary of a Credit
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             Support Obligation is seeking to enforce such Credit Support
             Obligation, the Majority Owned Subsidiary shall notify the
             Parent.  The Parent shall have the right, at its own
             expense, to contest the claim of such beneficiary.  If the
             Parent or the subsidiary of the Parent on whose behalf the
             Credit Support Obligation is given is contesting the claim
             of such beneficiary, the Majority Owned Subsidiary or
             wholly-owned subsidiary thereof, as applicable, will not
             perform under the relevant Credit Support Obligation unless
             and until, in the Majority Owned Subsidiary's reasonable
             judgment, the Majority Owned Subsidiary or wholly-owned
             subsidiary thereof, as applicable, is obligated under the
             terms of such Credit Support Obligation to perform.  Subject
             to the foregoing, any dispute between the Parent or the
             subsidiary of the Parent on whose behalf the Credit Support
             Obligation was given, on the one hand, and a beneficiary of
             a Credit Support Obligation, on the other, shall not affect
             the Parent's obligation to promptly indemnify the Majority
             Owned Subsidiary or its wholly-owned subsidiary, as
             applicable, hereunder.  

        4.   Upon the request of a Majority Owned Subsidiary, the Parent
             may make loans and advances to the Majority Owned Subsidiary
             or its wholly-owned subsidiaries on a short-term, revolving
             credit basis, from time to time in such amounts as mutually
             determined by the Parent and the Majority Owned Subsidiary.
             The aggregate principal amount of such loans and advances
             shall be reflected on the books and records of the Majority
             Owned Subsidiary (or wholly-owned subsidiary, as applicable)
             and the Parent.  All such loans and advances shall be on an
             unsecured basis unless specifically provided otherwise in
             loan documents executed at that time.  The Majority Owned
             Subsidiary or its wholly-owned subsidiaries, as applicable,
             shall pay interest on the aggregate unpaid principal amount
             of such loans from time to time outstanding at a rate
             ("Interest Rate") equal to the rate of the Commercial Paper
             Composite Rate for 90-day maturities as reported by Merrill
             Lynch Capital Markets, as an average of the last five
             business days of such Majority Owned Subsidiary's latest
             fiscal quarter then ended, plus twenty-five (25) basis
             points.  The Interest Rate shall be adjusted on the first
             business day of each fiscal quarter of such Majority Owned
             Subsidiary pursuant to the Interest Rate formula contained
             in the preceding sentence and shall be in effect for the
             entirety of such fiscal quarter.  Interest shall be computed
             on a 360-day basis.  The aggregate principal amount
             outstanding and accrued interest thereon shall be payable on
             demand.  The principal and accrued interest may be paid by
             the Majority Owned Subsidiaries or their wholly-owned
             subsidiaries, as applicable, at any time or from time to
             time, in whole or in part, without premium or penalty.  All
             payments shall be applied first to accrued interest and then
             to principal.  Principal and interest shall be payable in
             lawful money of the United States of America, in immediately

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             available funds, at the principal office of the Parent or at
             such other place as the Parent may designate from time to
             time in writing to the Majority Owned Subsidiary.  The
             unpaid principal amount of any such borrowings, and accrued
             interest thereon, shall become immediately due and payable,
             without demand, upon the failure of the Majority Owned
             Subsidiary or its wholly-owned subsidiary, as applicable, to
             pay its debts as they become due, the insolvency of the
             Majority Owned Subsidiary or its wholly-owned subsidiary, as
             applicable, the filing by or against the Majority Owned
             Subsidiary or its wholly-owned subsidiary, as applicable, 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 Majority Owned
             Subsidiary or its wholly-owned subsidiary, as applicable, 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 Majority Owned Subsidiary
             or its wholly-owned subsidiary, as applicable.  In case any
             payments of principal and interest shall not be paid when
             due, the Majority Owned Subsidiary or its wholly-owned
             subsidiary, as applicable, further promises to pay all cost
             of collection, including reasonable attorneys' fees.   

        5.   All payments required to be made by a Majority Owned
             Subsidiary or its wholly-owned subsidiaries, as applicable,
             shall be made within two days after receipt of notice from
             the Parent. All payments required to be made by the Parent
             shall be made within two days after receipt of notice from
             the Majority Owned Subsidiary.  

        6.   This Agreement shall be governed by and construed in
             accordance with the laws of the Commonwealth of
             Massachusetts applicable to contracts made and performed
             therein.

             IN WITNESS WHEREOF, the parties have caused this Agreement
        to be executed by their duly authorized officers as of the date
        first above written.


                                      THERMO INSTRUMENT SYSTEMS INC.

                                      By:  /s/ Melissa F. Riordan

                                      Title:    Treaurer


                                      THERMO OPTEK CORPORATION 


                                      By:  /s/ Robert J. Rosenthal

                                      Title:    President