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.
                       Promissory Note Due August 27, 1999
                                  Hurst, Texas


                                                                   March 3, 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 its assigns,  on or before August
27, 1999, the principal sum of two hundred  million  dollars  ($200,000,000)  or
such part thereof as then remains  unpaid.  The Company  shall pay interest from
the date hereof  through April 3, 1999 on the principal  amount  remaining  from
time to time  unpaid at a rate per  annum  equal to  5.03%.  Thereafter,  if the
Company's total U.S. consolidated cash invested with the Payee equals or exceeds
the aggregate  outstanding  principal amount of this Note, the Company shall pay
interest on the principal  amount  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 twenty-five (25) basis points, which
rate 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.  If,
however,  the Company's total U.S.  consolidated cash invested with the Payee is
less than the  aggregate  outstanding  principal  amount of this  Note,  (A) the
Company  shall pay  interest  at a rate per annum equal to the DCP Rate plus one
hundred twenty-five (125) basis points on that portion (the "Excess Portion") of
the unpaid  principal  amount equal to (i) the aggregate  outstanding  principal
amount, minus (ii) the Company's total U.S.  consolidated cash invested with the
Payee,  and (B) the Company  shall pay interest at a rate per annum equal to the
DCP Rate plus  twenty-five  (25)  basis  points on that  portion  of the  unpaid
principal amount equal to (i) the aggregate  outstanding principal amount, minus
(ii) the  Excess  Portion.  Each of the  interest  rates  set forth in the prior
sentence  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 third 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
announced  from time to time by  BankBoston  Corporation  at its head  office in
Boston,  Massachusetts  as its "base rate" plus one percent (1%).  Principal and
all accrued  but unpaid  interest  shall be due and payable on August 27,  1999.
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. The then unpaid principal amount of, and
interest outstanding on, this Note shall be

<PAGE>


                                      2
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.

      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.

<PAGE>

                                    THERMO INSTRUMENT SYSTEMS INC.


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

[Corporate Seal]

Attest:


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