THE MEASUREMENT OF RESISTANCE 



179 



narily used at the gaps c and d. The connections are then as 

 shown in Fig. 100. 



The contact q is placed at the zero of the slide wire and the 

 contact adjusted until balance is obtained. The coils are then 

 interchanged and the slider, q, is adjusted. Then 



BC 



A - 



B + C 



= resistance of section I* 



The coils are returned to their 

 original position, q remaining 

 fixed, and the contact ad- 

 justed until the balance is 

 again attained. The coils are 

 then interchanged and the bal- 

 ance obtained again by moving 

 the contact q. The slide wire 

 is thus divided into sections 

 each having a resistance, 



A - 



FIG. 100. Diagram for Carey Foster 

 method for calibrating a slide wire. 



BC 



As this difference must remain constant, the coils employed 



should be of manganin. 

 Barus and Strouhal Method for Calibrating a Slide Wire. 



This method is a simple application of the projection of poten- 

 tials; the wire to be calibrated 

 is placed in parallel with a 

 series of movable coils having 

 nearly equal resistances. For 

 convenience they may be 

 ranged along a board parallel 

 to the slide wire with their 



FIG. 101. Diagram for Barus and terminals dipping into mer- 

 Strouhal method for calibrating a ____ _____ 



slide wire. 



One of the coils, a, is selected 



as a standard, the galvanometer attached to one terminal of 

 it and balance obtained by use of the slider. The galvanometer 

 wire is then transferred to the other terminal of the coil and a 

 second balance obtained by use of the slider. Coil a is now in- 

 terchanged with coil b and the operation repeated. Thus, by 



