Measuring the Resistance of a Battery. 5 



Now, if we make the resistance of ADC large compared 

 with that of AB, the current in AB will be small compared 

 with the greatest current which the battery can produce. 

 Hence Lodge's method, although very valuable for many 

 purposes, seems unsuited for measuring the resistance of a 

 battery when producing a current of magnitude comparable 

 with that which passes through it when short-circuited. 



The method which I submit to your notice this evening 

 consists in replacing the galvanometer used in Mance's 

 method by the primary wire of a small induction coil, with 

 the secondary wire of which a telephone is connected. Con- 

 tact in BD is alternately made and broken rapidly by a 

 vibrating spring, kept in motion by an electro-magnetic 

 arrangement worked by a separate battery, or by the current 

 in BD itself, if none of the branches have sensible co- 

 efficients of self-induction, or the resistance of BD is 

 slightly varied by introducing into it a microphone, on 

 the stand of which a small clock is placed. B}^ either of 

 these methods we may produce periodic changes in the 

 resistance of BD, and make these changes so small that the 

 resistance and electro-motive force of AB will be practically 

 constant. Although the changes in the strength of the 

 current in AC will also be slight, yet the telephone will 

 detect alterations which could not be rendered evident by 

 galvanometric methods. 



The modification of Mance's method which I propose 

 seems to possess the two following advantages : — 



1. It enables us to measure the resistance of AB when a 

 powerful current is passing through it. 



2. In its empkvment the strength of current in AB need 

 be but slightly affected. 



For these reasons it appears to me especially suited for 

 measuring the resistance of an electric arc, the lamp being 

 introduced in the branch AB, the joint resistance of the 

 battery (or machine) and lamp measured, and the resistance 

 of machine, lamp fittings, and carbons separately ascertained. 



