1

                                                                     EXHIBIT (b)

                  LETTER AGREEMENT DATED MARCH 1, 2000 BETWEEN
                     THERMO ELECTRON AND THERMO INSTRUMENT

                                          March 1, 2000
Thermo Instrument Systems Inc.
81 Wyman Street
Waltham, MA 02454
Attn: Earl R. Lewis

Dear Earl:


     Reference is made to Thermo Instrument's proposed acquisitions of the
minority interest investment in each of its majority owned subsidiaries,
ThermoQuest Corporation, Thermo Optek Corporation, Thermo BioAnalysis
Corporation, Metrika Systems Corporation and ONIX Systems Inc. In connection
with those transactions, Thermo Electron Corporation intends to loan Thermo
Instrument up to $400,000,000 for the acquisition of such interests in each of
ThermoQuest Corporation, Thermo Optek Corporation, Thermo BioAnalysis
Corporation, Metrika Systems Corporation and ONIX Systems Inc. This loan will be
available at an interest rate equal to the 30-day Dealer Commercial Paper Rate
plus 150 basis points, set at the beginning of each month, provided such rate
shall be reduced to the Dealer Commercial Paper Rate plus 50 basis points to the
extent of any funds invested by Thermo Instrument's majority owned subsidiaries
in Thermo Electron's cash management arrangement. Interest will be due and
payable on a monthly basis and the loan will be payable in full on September 1,
2000. If these terms are acceptable to you, please sign the letter as indicated
below to evidence your acceptance of the terms.


                                          Sincerely,

                                          THERMO ELECTRON CORPORATION

                                          /s/ THEO MELAS-KYRIAZI
                                          Theo Melas-Kyriazi
                                          Vice President and Chief Financial
                                          Officer

ACCEPTED AND AGREED TO:

THERMO INSTRUMENT SYSTEMS INC.

/s/ EARL R. LEWIS
- --------------------------------------
Earl R. Lewis
President and CEO