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.


                         THERMO INSTRUMENT SYSTEMS INC.
           Amended and Restated Promissory Note Due February 28, 2000
                             Waltham, Massachusetts

                                                                 August 27, 1999

      For value received, Thermo Instrument Systems Inc., a Delaware corporation
(the  "Company"),   hereby  promises  to  pay  to  Thermo  Electron  Corporation
(hereinafter referred to as the "Payee"), or registered assigns, on February 28,
2000, the principal sum of one hundred fifty million dollars  ($150,000,000)  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 Dealer  Commercial Paper Rate for 30-day  maturities
as reported in The Wall Street  Journal on the first business day of each fiscal
month of the Company (the "DCP Rate") plus one-hundred fifty (150) basis points.
If, however, one or more of the Company's majority-owned U.S. subsidiaries (i.e.
not  wholly-owned)  is  participating  in the cash management  arrangement  with
Payee,  then the rate payable on the  Company's  outstanding  principal  balance
shall be  calculated  as  follows:  If the  aggregate  amount  of the  Company's
majority-owned  U.S.  subsidiaries'  cash  balance  under  the  cash  management
arrangement  ("Majority-Owned  Excess  Cash")  equals or exceeds  the  Company's
outstanding  principal  balance,  then the  Company  shall pay  interest  on the
aggregate unpaid principal amount hereunder at a rate per annum equal to the DCP
Rate plus fifty (50) basis points. If the aggregate amount of the Majority-Owned
Excess Cash is less than the Company's  outstanding  principal balance, then (A)
the  Company  shall pay  interest at a rate per annum equal to the DCP Rate plus
fifty (50) basis points on that portion of the unpaid  principal amount equal to
the Majority-Owned Excess Cash, and (B) the Company shall pay interest at a rate
per annum  equal to the DCP Rate plus one hundred  fifty  (150) basis  points on
that  portion  of  the  unpaid  principal  amount  equal  to (i)  the  Company's
outstanding  principal balance,  minus (ii) the Majority-Owned Excess Cash. Each
of the interest rates set forth in the prior  sentences shall be adjusted on the
second  business  day of each fiscal month of the Company and shall be in effect
for the  entirety  of such fiscal  month.  Interest is payable in arrears on the
first  business  day of each  fiscal  month of the  Company,  until all  amounts
outstanding are paid in full. Overdue principal and interest shall bear interest
at a rate per annum equal to the rate of interest published from time to time in
The Wall Street Journal as the "prime rate" plus one percent (1%). Principal and
all accrued but unpaid interest shall be repaid on February 28, 2000.  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

<PAGE>

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.

      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.

      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.

<PAGE>


      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 amends and restates in its entirety that certain promissory note
issued by the Company to Thermo Electron Corporation dated March 3, 1999.

      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.


                                    THERMO INSTRUMENT SYSTEMS INC.


                                    By:   /s/Earl R. Lewis
                                          ---------------------
                                          Earl R. Lewis
                                          President and CEO

[Corporate Seal]

Attest:



/s/Sandra L. Lambert
-----------------------
Sandra L. Lambert
Secretary