EXHIBIT 4.3



                    (FORM OF FACE OF REGISTERED SECURITIES)

                        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. R-
      __________

Cusip: 883559AE6
      __________
 
         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 ____________________ or registered assigns the principal sum
of _______ Dollars plus any premium, if any, on January 15, 2005 (the "Stated
Maturity") upon the presentation and surrender hereof 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 in whose
name this Security (or one or more Predecessor Securities) is registered at the
close of business on the Regular Record Date for such interest, which shall be
the January 1 or July 1 (whether or not a Business Day), as the case may be,
next preceding such Interest Payment Date at the Office or Agency of the Company
maintained for such purpose; provided, however, that such interest may be paid,
at the Company's option, by mailing a check to such Holder at its registered
address or by transfer of funds to an account maintained by such Holder within
the United States. Any such interest not so punctually paid or duly provided for
shall forthwith cease to be payable to the Holder on such Regular Record Date,
and may be paid to the Holder in whose name this Security (or one or more
Predecessor Securities) is registered at the close of business on a Special
Record Date for the payment of such Defaulted Interest to be fixed by the
Trustee, notice whereof shall be given to Holders of Securities of this series
not less than 10 days prior to such Special Record Date, or may be paid at any
time in any other lawful manner not inconsistent with the requirements of any
securities exchange on which the Securities of this series may be listed, and
upon such notice as may be required by such exchange, all as more fully provided
in the Indenture. Interest will be computed on the basis of a 360-day year of
twelve 30-day months. All payments hereunder shall be made in United States
Dollars.

 
         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 REVERSE OF REGISTERED 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 Agents 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.

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

                                      -4-

 
         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 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.

                                      -5-

 
         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 in the name
of 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 and 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
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

                                      -6-

 
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.

                                      -7-

 
                                   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

                                      -8-

 
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: _____________________________

                                      -9-

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

Dated:  _______________________                   BANKERS TRUST COMPANY,
                                                  as Trustee



                                                  By: __________________________
                                                      Authorized Officer

                                      -10-

 
                                TRANSFER NOTICE


FOR VALUE RECEIVED, the undersigned Holder hereby sell(s), assign(s) and
transfer(s) unto______________________________________________________________
______________________________________________________________________________
whose taxpayer identification number is _____________________ and whose address
including postal/zip code is _____________________________________ the within
Security and all rights thereunder, hereby irrevocably constituting and
appointing _____________________________ attorney-in-fact to transfer said
Security on the books of the Security Registrar with full power of substitution
in the premises.


Dated:___________________                            Name:______________________

                                                     By:________________________

                                                     Title:_____________________

                                                     NOTICE: The signature of
                                                     the Holder to this
                                                     assignment must correspond
                                                     with the name as written
                                                     upon the face of the within
                                                     instrument in every
                                                     particular, without
                                                     enlargement or any change
                                                     whatsoever.

                                                     SIGNATURE GUARANTEED

                                                     ___________________________

                                      -11-

 
                               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 and be registered in the name of the
undersigned unless a different name has been indicated below. If shares or
Securities 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
                                             [MUST BE GUARANTEED IF STOCK
                                             OR SECURITIES ARE TO BE ISSUED IN
                                             A NAME OTHER THAN THE  REGISTERED
                                             HOLDER OF THE SECURITY]


- ------------------------------------
               Name

- ------------------------------------
               Address

- ------------------------------------
Social Security or other Taxpayer
Identification Number, if any

                                      -12-

 
If shares or Registered Securities are to    If only a portion of the
be registered in the name of a Person        Securities is to be converted,
other than the Holder, print such            please please indicate:   
Person's name and address:    


                                             1. Principal amount to be
                                                converted:

                                             U.S. $___________________

_______________________________              2.  Principal amount and
           Name                                  denomination of Registered
                                                 Securities representing
                                                 unconverted principal amount
                                                 to be issued.
_______________________________
           Address
                                             Amount:  U.S.$_________________
_______________________________
Social Security or other Taxpayer            Denominations:
Identification Number, if any                U.S.$__________
                                             (any integral multiple of U.S.
                                             $1,000)

_______________________________

                                      -13-

 
                           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*(s)

                                                 Name: _______________________
                                                 Address:_____________________
                                                         _____________________
                                                 [* Signature Guarantee 
                                                 Required]

                                                 Principal amount to be repaid
                                                 (if less than all):

                                                 $__________

                                                 NOTICE: If this Security is
                                                 issued in registered form, the
                                                 above signatures of the
                                                 holder(s) hereof must
                                                 correspond with the name as it
                                                 appears in the Security in
                                                 every particular without
                                                 alteration or enlargement or
                                                 any change whatever.


____________________________
Social Security or Other
Taxpayer Identification Number

                                      -14-