Filer: Thermo Electron Corporation
                                 Subject Company: Thermo Instrument Systems Inc.
                                   Subject Company Exchange Act File No.: 1-9786
                                           Registration Statement No.: 333-35478
PRESS RELEASE:
Investor Contact: 781-622-1111
Media Contact: 781-622-1252

                            THERMO ELECTRON COMMENCES
                   EXCHANGE OFFER FOR THERMO INSTRUMENT SHARES

WALTHAM, Mass., April 24, 2000 - Thermo Electron Corporation (NYSE-TMO)
announced today that it has commenced its exchange offer to acquire all
outstanding shares of Thermo Instrument Systems Inc. (ASE-THI) common stock.
Under the exchange offer, Thermo Electron will exchange 0.85 shares of Thermo
Electron common stock for each Thermo Instrument share. The exchange offer is
scheduled to expire at midnight on May 19, 2000, unless the offer is extended.

         Thermo Electron currently owns approximately 88.6 percent of the
outstanding shares of Thermo Instrument common stock. The goal of the exchange
offer is to bring Thermo Electron's equity ownership in Thermo Instrument to at
least 90 percent. If Thermo Electron achieves this 90-percent ownership
threshold, it plans to acquire all remaining outstanding shares of Thermo
Instrument common stock through a "short-form" merger. In the short-form merger,
public shareholders who do not participate in the exchange offer would also
receive shares of Thermo Electron common stock at the same exchange ratio that
is being offered in the exchange offer.

         The exchange offer and proposed subsequent short-form merger require
Securities and Exchange Commission clearance of necessary filings. The
short-form merger would not require Thermo Instrument board or shareholder
approval.

         Upon completion of the exchange offer and the merger, Thermo Electron
would assume the outstanding obligations under Thermo Instrument's convertible
debentures, and these debentures would become convertible into shares of Thermo
Electron common stock.

         Thermo Electron is mailing today to registered holders of Thermo
Instrument shares a preliminary prospectus regarding the exchange offer and the
merger, accompanied by a letter of transmittal that Thermo Instrument
shareholders can use to tender their shares in the exchange offer. Beneficial
owners of Thermo Instrument shares holding in street name through their brokers
can receive the preliminary prospectus and letter of transmittal from their
broker and should contact their brokers for those documents. In any case, copies
of the preliminary prospectus and letter of transmittal may be requested from
D.F. King & Co., Inc., the information agent for the exchange offer, by calling
(800) 290-6433.

         Thermo Electron Corporation is a leading provider of analytical and
monitoring instruments used in a broad range of applications, from life sciences
research to telecommunications to food and beverage production. In addition,
Thermo Electron serves the healthcare market through a family of medical
products, and is a major producer of paper recycling systems and provides
water-clarification and fiber-recovery products and services. As announced on
January 31, 2000, the company has initiated a major reorganization that would
transform it into one company focused on its core measurement and detection
instruments business. More information is available on the Internet at
http://www.thermo.com.

                                     -more-






OTHER IMPORTANT INFORMATION:

WE URGE  INVESTORS AND SECURITY  HOLDERS TO READ THE FOLLOWING  DOCUMENTS,  WHEN
THEY BECOME AVAILABLE,  REGARDING THE EXCHANGE OFFER AND THE MERGER DESCRIBED IN
THIS ANNOUNCEMENT, BECAUSE THEY CONTAIN IMPORTANT INFORMATION:

*   THERMO ELECTRON'S PRELIMINARY PROSPECTUS, ANY PROSPECTUS SUPPLEMENT, FINAL
    PROSPECTUS, AND OTHER EXCHANGE OFFER MATERIALS.

*   THERMO ELECTRON'S  REGISTRATION  STATEMENT ON FORM S-4 AND SCHEDULE TO, AS
    AMENDED,  WHICH CONTAIN OR  INCORPORATE  BY REFERENCE THE DOCUMENTS  LISTED
    ABOVE AND OTHER INFORMATION.

*   THERMO INSTRUMENT'S SOLICITATION/RECOMMENDATION STATEMENT ON SCHEDULE 14D-9.

THESE  DOCUMENTS AND  AMENDMENTS TO THESE  DOCUMENTS  HAVE BEEN OR WILL BE FILED
WITH THE SEC. WHEN THESE AND OTHER DOCUMENTS ARE FILED WITH THE SEC, THEY MAY BE
OBTAINED FOR FREE AT THE SEC'S WEB SITE AT www.sec.gov.  YOU MAY ALSO OBTAIN FOR
FREE EACH OF THESE DOCUMENTS,  WHEN AVAILABLE, FROM US. YOU CAN CALL US AT (781)
622-1111 OR WRITE TO US AT:

INVESTOR RELATIONS DEPARTMENT
THERMO ELECTRON CORPORATION
81 WYMAN STREET, P.O. BOX 9046
WALTHAM, MA  02454-9046


The following constitutes a "Safe Harbor" statement under the Private Securities
Litigation Reform Act of 1995: This press release contains forward-looking
statements that involve a number of risks and uncertainties. Important factors
that could cause actual results to differ materially from those indicated by
such forward-looking statements are set forth under the heading "Forward-Looking
Statements" in Exhibit 13 to the company's annual report on Form 10-K, for the
year ended January 1, 2000. These include risks and uncertainties relating to:
the company's corporate reorganization, acquisition strategy, growth strategy,
international operations, product development and technological change, possible
changes in governmental regulations, changes in both capital spending by
commercial customers and government funding policies, and dependence on
intellectual property rights.


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