EXHIBIT 4.5
                                                                                
               FORM OF CONVERTIBLE SUBORDINATED BEARER SECURITY
                              [Face of Security]

ANY UNITED STATES PERSON WHO HOLDS THIS SECURITY WILL BE SUBJECT TO LIMITATIONS
UNDER THE UNITED STATES INCOME TAX LAWS, INCLUDING THE LIMITATIONS PROVIDED IN
SECTIONS 165(j) AND 1287(a) OF THE UNITED STATES INTERNAL REVENUE CODE.

                         THERMO INSTRUMENT SYSTEMS INC.
                    (INCORPORATED IN THE STATE OF DELAWARE)


                     4% CONVERTIBLE SUBORDINATED DEBENTURE
                             DUE JANUARY 15, 2005
                     GUARANTEED ON A SUBORDINATED BASIS BY
                          THERMO ELECTRON CORPORATION
                    (INCORPORATED IN THE STATE OF DELAWARE)

NO. ____________                                                   $____________

          Thermo Instrument Systems Inc., a Delaware corporation (herein
referred to as the "Company," which term includes any successor corporation
under the Indenture referred to on the reverse hereof), for value received,
hereby promises to pay to bearer upon presentation and surrender hereof the
principal sum of $_______ United States Dollars plus any premium, if any, on
January 15, 2005 (the "Stated Maturity") or, at the option of the Company, in
whole or in part at any time on or after January 15, 2001, upon notice as
described in the Indenture and to pay interest thereon from January 21, 1998 or
from the most recent Interest Payment Date to which interest has been paid or
duly provided for, semi-annually in arrears on January 15 and July 15 in each
year (each, an "Interest Payment Date"), commencing July 15, 1998, at the rate
of 4% per annum, and Additional Amounts, if any, until the principal hereof is
paid or duly provided for. The interest and Additional Amounts, if any, so
payable, and punctually paid or duly provided for, on any Interest Payment Date
will, as provided in such Indenture, be paid to the Holder surrendering the
appropriate Coupon appertaining hereto. Interest will be computed on the basis
of a 360-day year of twelve 30-day months.

          Such payments (including premium, if any) shall be made in United
States Dollars.  The Company shall maintain, subject to any laws or regulations
applicable thereto, an Office or Agency in a Place of Payment that is located
outside the United States where this Security and any Coupons appertaining
hereto, and Guarantees with respect hereto, may be presented and surrendered for
payment.  Subject to the right of the Company (limited as provided in the
Indenture) to change the location of any Office or Agency, the  Place of Payment
and Place of Conversion with respect to this Security shall be either at the
London office of Bankers Trust Company located at 1 Appold Street, Broadgate, 
London, EC2A 2HE, England, at Bankers Trust 


 
Luxembourg, S.A. located at 14 Boulevard, F.D. Roosevelt, L-2450 Luxembourg or
at such other Offices or Agencies outside the United States as the Company may
designate. Such payments shall be made by United States Dollar check drawn on a
bank in the City of New York, or (at the option of the Company with the consent
of the Paying Agent) by transfer to a United States Dollar account maintained by
the Holder at a bank located outside the United States. Interest on this
Security shall be paid only at an Office or Agency located outside the United
States and, in the case of interest due on or before Maturity, only upon
presentation and surrender at such an Office or Agency of the applicable
interest Coupons hereto attached as they severally mature. No payment on this
Security or any Coupon will be made at the Corporate Trust Office of the Trustee
or any other Paying Agent maintained by the Company in the United States, nor
will any payment be made by transfer to an account in the United States, except
as may be permitted by United States tax laws and regulations in effect at the
time of such payment without detriment to the Company. Notwithstanding the
foregoing, payment of this Security and Coupons may be made at the Corporate
Trust Office of the Trustee in the City of New York if full payment at all
Paying Agents outside the United States is illegal or effectively precluded by
exchange controls or other similar restrictions.

          Reference is hereby made to the further provisions of this Security
set forth on the reverse hereof, which further provisions shall for all purposes
have the same effect as if set forth at this place.

          Unless the Certificate of Authentication hereon has been executed by
the Trustee by manual signature of one of its authorized signatories, this
Security shall not be entitled to any benefit under the indenture, or be valid
or obligatory for any purpose.

                                      -2-

 
    IN WITNESS WHEREOF, the Company has caused this instrument to be duly
executed under its facsimile corporate seal.

Dated: January __, 1998                     THERMO INSTRUMENT SYSTEMS INC.
                                            
                                            By:_______________________
Attest:                                        Name
                                               Title
_______________________________
Secretary

[SEAL]

                         CERTIFICATE OF AUTHENTICATION

  This is one of the Securities of the series designated therein referred to in
the within-mentioned Indenture.

                              BANKERS TRUST COMPANY,
                              As Trustee

                                 BY:_____________________________
                                    Authorized Officer

                                      -3-

 
                 (FORM OF FACE OF COUPON ON BEARER SECURITIES)

ANY UNITED STATES PERSON WHO HOLDS THIS SECURITY WILL BE SUBJECT TO LIMITATIONS
UNDER THE UNITED STATES INCOME TAX LAWS, INCLUDING THE LIMITATIONS PROVIDED IN
SECTIONS 165(j) AND 1287(a) OF THE UNITED STATES INTERNAL REVENUE CODE.

                         THERMO INSTRUMENT SYSTEMS INC.
                    (Incorporated in the State of Delaware)

                4% CONVERTIBLE SUBORDINATED DEBENTURE DUE 2005

                                              No:  _____________

                                              U.S.$____________

                                              Due:

          Unless the Security to which this Coupon appertains shall have been
called for redemption or presented for repayment prior to the due date hereof
and payment thereof duly provided for or shall have been converted, Thermo
Instrument Systems Inc. (herein called the "Company") shall, subject to and in
accordance with the terms and conditions of the Bearer Security and the
Indenture dated as of January 15, 1998 by the Company, as issuer, and Thermo
Electron Corporation, as guarantor, to Bankers Trust Company, as Trustee, pay to
the bearer, on the date set forth herein upon surrender hereof, the amount shown
hereon (together with any Additional Amounts in respect thereof which the
Company may be required to pay according to the terms of said Indenture) at the
offices of the Paying Agent set out on the reverse hereof or at such other
places outside the United States as the Company may determine from time to time,
by United States Dollar check drawn on a bank in the City of New York, or (at
the option of the Company with the consent of the Paying Agent) by transfer to a
United States Dollar account maintained by the bearer at a bank outside the
United States, being one-half year's interest then payable on said Security.

                                              THERMO INSTRUMENT SYSTEMS INC.


                                              By:___________________________
                                                 Name:
                                                 Title:
Attest:

__________________________

                                      -4-

 
                              [Reverse of Coupon]


Bankers Trust Company                         Bankers Trust Luxembourg, S.A.
1 Appold Street                               14 Boulevard, F.D. Roosevelt
Broadgate, London EC2A 2HE                    L-2450 Luxembourg
England

                                      -5-

 
                     (FORM OF REVERSE OF BEARER SECURITIES)

                         THERMO INSTRUMENT SYSTEMS INC.


          This Security is one of a duly authorized issue of securities of the
Company (herein called the "Securities"), issued and to be issued in one or more
series under an Indenture, dated as of January 15, 1998 (herein called the
"Indenture") by the Company, as issuer, and Thermo Electron Corporation, as
guarantor (herein called the "Guarantor," which term includes any successor
corporation under the Indenture), to Bankers Trust Company, as Trustee (herein
called the "Trustee," which term includes any successor trustee under the
Indenture with respect to the series of which this Security is a part), to which
Indenture and all indentures supplemental thereto reference is hereby made for a
statement of the respective rights, limitations of rights, duties and immunities
thereunder of the Company, the Guarantor, the Trustee and the Holders of the
Securities and any Coupons appertaining thereto, and of the terms upon which the
Securities are, and are to be, authenticated and delivered. This Security is one
of the duly authorized series of Securities designated on the face hereof and
the aggregate principal amount of the Securities to be issued under such series
is limited to $250,000,000 (except for Securities authenticated and delivered
upon transfer of, or in exchange for, or in lieu of other Securities). The
Guarantor has unconditionally guaranteed on a subordinated basis the payment of
principal, premium, if any, and interest on the Securities. The Holders of the
Securities will be entitled to the benefits of, be bound by, and be deemed to
have notice of, all of the provisions of the Indenture. A copy of the Indenture
is on file and may be inspected at the office of the Paying Agent appointed by
the Company. All terms used in this Security which are defined in the Indenture
shall have the meanings assigned to them in the Indenture.

          The Securities are issuable as bearer securities (the "Bearer
Securities"), with interest Coupons attached, in the denominations of U.S.
$1,000 AND U.S. $10,000, and as registered securities (the "Registered
Securities"), without Coupons, in denominations of U.S. $1,000 and integral
multiples thereof, except as otherwise provided in or pursuant to the Indenture.
The Registered Securities, and transfers thereof, shall be registered as
provided in the Indenture.  The holder of any Bearer Security or any Coupon
appertaining thereto and the registered holder of a Registered Security shall
(to the fullest extent permitted by applicable law) be treated at all times, by
all persons and for all purposes as the absolute owner of such Security or
Coupon, as the case may be, regardless of any notice of ownership, theft or loss
or of any writing thereon.  Bearer Securities may be exchanged for Registered
Securities as set forth in the Indenture.  Registered Securities may not be
exchanged for Bearer Securities.

          The Securities are general, unsecured obligations of the Company,
subordinated in right of payment to the prior payment in full of all existing
and future Senior Indebtedness of the Company to the extent set forth in Article
Seventeen of the Indenture.  There are no restrictions herein on other
indebtedness or securities which may be incurred or issued by the Company or the
Guarantor.

          The Securities have been approved for listing on the American Stock 
Exchange (the "AMEX"). Contracts for Securities traded on the AMEX must be 
settled in Registered Securities.

          Additional Amounts shall be payable on the Securities as set forth in
the Indenture.

                                      -6-

 
          If an Event of Default, as defined in the indenture, shall occur and
be continuing, the principal of the Securities of this series may be declared
due and payable in the manner and with the effect provided in the Indenture.

          Subject to the terms of the Indenture, the Securities are convertible
at any time prior to Maturity at an initial Conversion Price of $35.65 per 
share, subject to adjustment as provided in the Indenture. Accrued interest from
the immediately preceding Interest Payment Date until the Conversion Date will
be paid within five Business Days after the Conversion Date. In case this
Security, or a portion thereof, is called for redemption, or is delivered for
repayment, the conversion right in respect of this Security or a portion thereof
so called and delivered shall expire on the close of business on the fifth
Business Day preceding the Redemption Date or the second Business Day preceding
a repayment on the Repayment Date, as the case may be, unless the Company
defaults in making the payment due upon redemption or repayment, as the case may
be. To convert this Security, the Holder must surrender this Security, duly
endorsed or assigned to the company, at any applicable place of conversion,
accompanied by a duly signed conversion notice. Bearer Securities surrendered
for conversion must be surrendered together with all Coupons appertaining
thereto that mature after the Conversion Date, except as otherwise provided in
the Indenture.

          The Securities are subject to redemption at any time on or after
January 15, 2001, as a whole or in part, at the option of the Company, at a
Redemption Price equal to 100% of the principal amount, together in the case of
any such redemption with accrued interest to the Redemption Date as provided in
the Indenture.  Notice of redemption will be given to Holders of Securities, not
less than 20 nor more than 60 days prior to the Redemption Date, all as provided
in the Indenture.  Subject to the terms of the indenture, if as a result of a
Tax Law Change the Company has or will become obligated to pay to the Holder of
any Tax Affected Security Additional Amounts, and such obligation cannot be
avoided by the Company taking reasonable measures available to it, then the
Company may, at its option, redeem the Tax Affected Securities as a whole, but
not in part, upon not less than 20 nor more than 60 days' notice to the Holders
prior to the Redemption Date therefor, at a Redemption Price equal to 100% of
the principal amount plus interest accrued to the Redemption Date, and any
Additional Amounts then payable.

          Subject to the terms of the Indenture, the Securities are subject to
repayment at the option of the Holder thereof, in whole or in part, on the
Repayment Date, if the Common Stock (or other equity securities into which this
Security is then convertible) is neither listed for trading on a United States
national securities exchange, the Nasdaq National Market nor approved for
trading on an established automated over-the-counter trading market in the
United States. In such event, the Securities shall be repaid at a price equal to
the principal amount hereof, plus accrued interest 

                                      -7-

 
and Additional Amounts, if any. The Holders of Securities of this series
desiring to be repaid must deliver (1) this Security with the "Option to Elect
Repayment" duly signed and completed, or (2) such other documentation as may be
provided by the Indenture, to the Trustee at the Place of Payment not earlier
than 60 days nor later than 30 days prior to the Repayment Date. Bearer
Securities must be surrendered together with all Coupons appertaining thereto
that mature after the Repayment Date, except as otherwise provided in the
Indenture.

     In the event of redemption, exchange, repayment or conversion of this
Security in part only, a new Security or Securities for the unredeemed,
unexchanged, unrepaid or unconverted portion hereof shall be issued to the
Holder hereof upon the cancellation hereof as set forth in the Indenture.

     The Place of Payment, place of registration of transfer and exchange and
the Place of Conversion for the Registered Securities and the Guarantees
endorsed thereon, shall be the Borough of Manhattan, The City of New York, and
the Corporate Trust Office of the Trustee shall be the Company's and the
Guarantor's Office or Agency in the Borough of Manhattan, The City of New York
for such purpose.  The Places of Payment and Places of Conversion for the Bearer
Securities and the Guarantees endorsed thereon, shall be London, England and
Luxembourg, and the offices of the Trustee in London and Luxembourg shall be the
Offices or Agencies for such purpose and the offices of the Trustee in London
and Luxembourg shall be the Company's and the Guarantor's Paying Agent in London
and Luxembourg, respectively.

     The Trustee initially shall be the Security Registrar, one of the
Authenticating Agents, the Paying Agent and the Conversion Agent.  The Trustee
presently has its Corporate Trust Office at Four Albany Street, 4th floor, New
York, NY 10006, its Office in London at 1 Appold Street, Broadgate, London, EC2A
2HE, England and its Office in Luxembourg at 14 Boulevard, F.D. Roosevelt, L-
2450 Luxembourg. The Company and the Guarantor may subsequently appoint an
Office or Agency in addition to or other than the Borough of Manhattan, The City
of New York with respect to the Registered Securities and London, England and
Luxembourg with respect to the Bearer Securities.

     The Indenture permits, with certain exceptions as therein provided, the
amendment thereof and the modification of the rights and obligations of the
Company and the Guarantor and the rights of the Holders of the Securities under
the Indenture at any time by the Company, the Guarantor and the Trustee with the
consent of the Holders of not less than a majority of the aggregate principal
amount of all Securities issued under the Indenture at the time Outstanding and
affected thereby. The Indenture also contains provisions permitting the Holders
of not less than a majority of the aggregate principal amount of the Outstanding
Securities, on behalf of the Holders of all such Securities, to waive compliance
by the Company or the Guarantor with certain provisions of the Indenture.
Furthermore, provisions in the Indenture permit the Holders of not less than a
majority of the aggregate principal amount, in certain instances, of the
Outstanding Securities of any series to waive, on behalf of all of the Holders
of Securities of such series, certain past defaults under the Indenture and
their consequences. Any such consent or waiver by the Holder of this Security
shall be conclusive and binding upon such Holder and upon all future Holders of
this Security and other Securities issued upon the registration of transfer

                                      -8-

 
hereof or in exchange herefor or in lieu hereof, whether or not notation of such
consent or waiver is made upon this Security.

     No reference herein to the indenture and no provision of this Security or
of the Indenture shall alter or impair the obligation of the Company, which is
absolute and unconditional (except as to the specific subordination provisions
contained in Article Seventeen of the Indenture), to pay the principal of,
premium, if any, interest and Additional Amounts, if any, on this Security at
the times, places and rates, and in the coin or currency, herein prescribed.

     No service charge shall be made for any registration of transfer or
exchange, but the Company may require payment of a sum sufficient to cover any
tax or other governmental charge payable in connection therewith.

     The Indenture and the Securities shall be governed by and construed in
accordance with the laws of the Commonwealth of Massachusetts applicable to
agreements made and to be performed entirely in such Commonwealth except that
the rights, protections, obligations, indemnities and immunities of the Trustee
under the Indenture and the Securities shall be governed by the laws of the
State of New York, without regard to the principles of conflicts of laws of
either state.

                                      -9-

 
                                   GUARANTEE

     For value received, Thermo Electron Corporation, a corporation organized
under the laws of the State of Delaware (herein called the "Guarantor," which
                                                            ---------        
term includes any successor corporation under the Indenture referred to in the
Security upon which this Guarantee is endorsed), hereby unconditionally
guarantees to the Holder of the Security upon which this Guarantee is endorsed
and to the Trustee on behalf of the Trustee and such Holder the due and punctual
payment of the principal of, premium, if any, and interest and Additional
Amounts, if any, on such Security, any other amount due and payable pursuant to
the terms of the Indenture and the due and punctual payment of the sinking fund
or analogous payments referred to therein if any, when and as the same shall
become due and payable, whether at the Stated Maturity, by declaration of
acceleration, call for redemption, repayment at the option of the Holder or
otherwise, according to the terms thereof and of the Indenture.  In case of the
failure of Thermo Instrument Systems Inc., a corporation organized under the
laws of Delaware (herein called the "Company," which term includes any successor
                                     -------                                    
corporation under such Indenture), punctually to make any such payment of
principal, premium, if any, or interest or Additional Amounts, if any, the
Guarantor hereby agrees to cause any such payment to be made punctually when and
as the same shall become due and payable, whether at the Stated Maturity or by
declaration of acceleration, call for redemption, repayment at the option of the
Holder or otherwise, and as if such payment were made by the Company.

     The Guarantor hereby agrees that its obligations hereunder shall be as if
it were principal debtor and not merely surety, and shall be absolute and
unconditional (except as to the specific subordination provisions contained in
Article Eighteen of the Indenture), irrespective of, and shall be unaffected by,
any invalidity, subordination, irregularity or unenforceability of such Security
or such Indenture, any failure to enforce the provisions of such Security or
such Indenture, or any waiver, modification or indulgence granted to the Company
with respect thereto, by the Holder of such Security or the Trustee or any other
circumstance which may otherwise constitute a legal or equitable discharge of a
surety or guarantor.  The Guarantor hereby waives the benefits of diligence,
presentment, demand of payment, filing of claims with a court in the event of
merger, insolvency or bankruptcy of the Company, any right to require a
proceeding first against the Company, protest or notice with respect to such
Security or the indebtedness evidenced thereby and all demands whatsoever, and
covenants that this Guarantee will not be discharged except by strict and
complete performance of the obligations contained in such Security and this
Guarantee.  The Guarantor hereby agrees that, in the event of a default in
payment of principal of, premium, if any, and interest, and Additional Amounts,
if any, on such Security, or default in any sinking fund or analogous payment
referred to therein, legal proceedings may be instituted by the Trustee on
behalf of, or by, the Holder of such Security, on the terms and conditions set
forth in the Indenture, directly against the Guarantor to enforce this Guarantee
without first proceeding against the Company.

     The Guarantor shall be subrogated to all rights of the Holder of such
Security and the Trustee against the Company in respect of any amounts paid to
such Holder by the Guarantor on account of such Security pursuant to the
provisions of this Guarantee or the Indenture; provided, however, that the
                                               --------  -------          
Guarantor shall not be entitled to enforce, or to receive any payments arising

                                      -10-

 
out of or based upon, such right of subrogation until the principal of, premium,
if any, and interest and Additional Amounts, if any, on all Securities issued
under such Indenture shall have been paid in full.

     Claims under this Guarantee are, to the extent provided in the Indenture,
subject in right of payment to the prior payment in full of all Senior Guarantor
Indebtedness, and this Guarantee is issued subject to the provisions of the
Indenture with respect thereto.  Each Holder of a Security upon which this
Guarantee is endorsed, by accepting the same, (a) agrees to and shall be bound
by such provisions, (b) authorizes and directs the Trustee on his behalf to take
such action as may be necessary or appropriate to effectuate the subordination
so provided and (c) appoints the Trustee his, her or its attorney-in-fact for
any and all such purposes.

     No reference herein to such Indenture and no provision of this Guarantee or
of such Indenture shall alter or impair the guarantee of the Guarantor, which is
absolute and unconditional (except as to the specific subordination provisions
contained in Article Eighteen of the Indenture), of the due and punctual payment
of principal, premium (if any), and interest and Additional Amounts, if any, on
the Security upon which this Guarantee is endorsed.

     This Guarantee shall not be valid or obligatory for any purpose until the
certificate of authentication of the Security upon which this Guarantee is
endorsed shall have been manually executed by or on behalf of the Trustee under
such Indenture.

     All terms used in this Guarantee which are defined in such Indenture shall
have the meanings assigned to them in such Indenture.

     This Guarantee shall be deemed to be a contract made under the laws of the
Commonwealth of Massachusetts, and for all purposes shall be governed by and
construed in accordance with the laws of the Commonwealth of Massachusetts.

     IN WITNESS WHEREOF, the Guarantor has caused this Guarantee to be duly
executed under its corporate seal and dated the date on the face hereof.

                                    THERMO ELECTRON CORPORATION


Attest:_______________________      By:________________________________


                                    Title: ____________________________

                                      -11-

 
     This is one of the Securities of the series designated therein referred to
in the within-mentioned Indenture.

                                    BANKERS TRUST COMPANY,
                                    as Trustee



                                    By: ______________________________
                                        Authorized Officer

                                      -12-

 
                               CONVERSION NOTICE


     The undersigned Holder of this Security hereby irrevocably exercises the
option to convert this Security, or any portion of the principal amount hereof
(which is an integral multiple of U.S. $1,000 below designated) into shares of
Common Stock in accordance with the terms of the Indenture referred to in this
Security and directs that such shares, together with a check in payment for any
fractional share and any Securities representing any unconverted principal
amount hereof, be delivered to the undersigned unless a different name has been
indicated below.  The address for payment of any such check must be outside the
United States.  If shares are to be registered in the name of a Person other
than the undersigned, the undersigned will pay all transfer taxes payable with
respect thereto.


Dated: __________________                        ______________________________
                                                 Signature

 
If shares are to be registered in the name of    HOLDER
and delivered to a Person other than the
Holder, please print such Person's name and      Please print name and address 
address:                                         of Holder 
 
 
 
_____________________________________          _________________________________
             Name                                         Name
 
_____________________________________          _________________________________
             Address                                      Address
 
_____________________________________          _________________________________
 
 
_____________________________________          _________________________________
 
 
_____________________________________          _________________________________
Social Security or other Taxpayer              Social Security or other Taxpayer
Identification Number, if any                  Identification Number, if any

                                      -13-

 
 Name and address (outside the United    If only a portion of the Securities 
 States) to where any check referred     is to be converted, please indicate:
 to in the first paragraph of this 
 Conversion Notice should be mailed:     1.  Principal amount to be converted:  
                                             U.S.  $______________
______________________________________   2.  Principal amount and denomination
Name                                         of Bearer Securities representing 
                                             unconverted principal amount to 
                                             be issued:  
______________________________________       Amount:  U.S. $_________________
Address                                  Denominations:  U.S. $________________
                                         (any integral multiple of U.S. $1,000)
 

                                      -14-

 
                           OPTION TO ELECT REPAYMENT
                             UPON A REPAYMENT EVENT


To:  Thermo Instrument Systems Inc.

     The undersigned owner of this Security hereby acknowledges receipt of a
notice from Thermo Instrument Systems Inc. (the "Company") as to the occurrence
of a Repayment Event with respect to the Common Stock of the Company and
requests and instructs the Company to repay the entire principal amount of this
Security, or the portion thereof (which is $1,000 or an integral multiple
thereof) below designated, in accordance with the terms of the Indenture
referred to in this Security at the Repayment Price (or the relevant percentage
thereof), together with accrued interest to such date (except if the Repayment
Date shall be an Interest Payment Date), to the holder hereof.


Dated: ________________________


                            ________________________________
                            Signature

                            Name: __________________________
                            Address:_________________________
                                    _________________________
                                Principal amount to be repaid
                            (if less than all):

                            $__________

 

_____________________________
Social Security or Other
Taxpayer Identification Number

                                      -15-