
<PAGE>
 
                                                                       Exhibit 4

        FIRST SUPPLEMENTAL INDENTURE, dated as of June 2, 1997 between AMETEK, 
Inc., a Delaware corporation (the "Company") and CoreStates Bank, N.A. (the 
"Trustee"), as Trustee under the Indenture hereinafter referred to.

        WHEREAS, the Company and the Trustee have previously entered into an 
Indenture dated as of March 15, 1994 (the "Indenture") relating to the 9 3/4% 
Senior Notes, due 2004, of the Company ("Notes");

        WHEREAS, Section 902 of the Indenture provides that the Company and the 
Trustee may, with the written consent of the holders of at least a majority in 
aggregate principal amount of the outstanding Notes, amend the Indenture as 
provided herein;

        WHEREAS, the holders of not less than a majority in aggregate principal 
amount of the outstanding Notes have consented to this First Supplemental 
Indenture; and

        WHEREAS, all acts and things prescribed by law and by the Company's 
Certificate of Incorporation and the By-Laws (each as now in effect) necessary 
to make this First Supplemental Indenture a valid instrument legally binding on 
the Company for the purposes herein expressed, in accordance with its terms, 
have been duly done and performed;
        
        NOW THERETOFORE, in consideration of the foregoing and for other good 
and valuable consideration, the receipt of which is hereby acknowledged, the 
Company and the Trustee hereby agree for the benefit of each other and the equal
and ratable benefit of the holders of the Notes as follows:

        1. Amendment of Article One, Section 101. The alphabetical list of 
           -------------------------------------
definitions included in Section 101 of the Indenture is hereby amended by 
inserting between the definitions of "Credit Agreement" and "Debt" the following
definition:
<PAGE>

          "Culligan Transaction" means, collectively, the 
          contributions, distribution and merger transactions 
          contemplated by that certain Amended and Restated 
          Agreement and Plan of Merger and Reorganization,
          dated as of February 5, 1997 and as thereafter from 
          time to time amended, by and among the Company, 
          Ametek Aerospace Products, Inc., a Delaware 
          corporation and a wholly owned subsidiary of the 
          Company, Culligan Water Technologies, Inc., a 
          Delaware corporation ("Culligan") and Culligan 
          Water Company, Inc., a Delaware corporation and 
          a wholly owned subsidiary of Culligan.

     2.   Amendment of Article Ten. Section 1010. Section 1010 of the Indenture 
          --------------------------------------
is hereby amended by inserting the following paragraph at the end of such 
Section:

               Notwithstanding the foregoing, the term 
          "Restricted Payments" shall not include the Culligan 
          Transaction or any securities, cash or other assets 
          issued, paid or distributed in connection therewith.

     3.  Construction. For all purposes of this First Supplemental Indenture, 
         ------------
except as otherwise herein expressly provided or unless the context otherwise 
requires: (i) the terms and expressions used herein shall have the same meanings
as corresponding terms and expressions used in the Indenture; and (ii) the words
"herein," "hereof" and "hereby" and other words of similar import used in this 
First Supplemental Indenture refer to this First Supplemental Indenture as a 
whole and not to any particular Section hereof.

     4.  Trustee Acceptance. The Trustee accepts the amendment of the Indenture 
         ------------------
effected by this First Supplemental Indenture and agrees to execute the trust 
created by the Indenture, as hereby amended, but only upon the terms and
conditions set forth in the Indenture, as hereby amended, including the terms
and provisions defining and limiting the liabilities and responsibilities of the
Trustee, which terms and provisions shall in like manner define and limit its
liabilities and responsibilities in the performance of the trust created by the
Indenture, as

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<PAGE>
 
hereby amended. Without limiting the generality of the foregoing, the Trustee 
has no responsibility for the correctness of the recitals of fact herein 
contained which shall be taken as the statements of the Company and makes no 
representations as to the validity, enforceability against the Company, or 
sufficiency of this First Supplemental Indenture.

        5.   Indenture Ratified.  Except as expressly amended hereby, the 
             ------------------
Indenture is in all respects ratified and confirmed and all the terms, 
conditions and provisions thereof shall remain in full force and effect.

        6.   Holders Bound.  This First Supplemental Indenture shall form a part
             -------------
of the Indenture for all purposes, and every Holder of the Notes heretofore or 
hereafter authenticated and delivered shall be bound hereby.

        7.   Successors and Assigns.  The First Supplemental Indenture shall be 
             ----------------------
binding upon and inure to the benefit of the parties hereto and their respective
successors and assigns.

        8.   Counterparts.  This First supplemental Indenture may be executed in
             ------------
any number of counterparts, each of which when so executed shall be deemed to be
an original, and all of such counterparts shall together constitute one and the 
same instrument.

        9.   Governing Law.   This First Supplemental Indenture shall be 
             -------------
governed by and construed in accordance with the laws of the State of New York.

                                       3
<PAGE>
 
        IN WITNESS WHEREOF, the Company and the Trustee have caused this First 
Supplemental Indenture to be signed and executed as of the day and year first 
above written.

                                       AMETEK, INC.


                                       By: /s/ Deirdre D. Saunders
                                           -----------------------
                                       Name: Deirdre D. Saunders
                                       Title: Treasurer and Assistant Secretary


                                       CORESTATES BANK, N.A.


                                       By: /s/ J. Bruce Herd
                                           ------------------
                                       Name: J. Bruce Herd
                                       Title: Vice President











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