ELECTRICAL MEASUREMENTS. 



28l 



so that the ratio 7/6 may have a series of values any one of 

 which may be chosen at will. The portion @ contains usually 

 I, 2, 2, and 5 of each units, tens, hundreds, etc., of ohms. An 

 unknown resistance a is connected as shown, the ratio 7/8 is 

 chosen, and the value of ft is changed until the galvanometer 



Fig. 208. 



gives no deflection, the battery key K' being closed first and 

 the galvanometer key K afterwards.* The value of a. is then 

 calculated with the help of equation (82). 



157. The measurement of resistance by the ammeter and volt- 

 meter. In the dynamo testing laboratory, where it is usually 

 inconvenient to use Wheatstone's bridge, resistance is ordinarily 

 measured by means of an ammeter and a voltmeter as follows : A 

 current, which is measured by an ammeter, is sent through the 

 resistance, and the electromotive force between the terminals of 

 the resistance is measured by means of a voltmeter. The value 



* If the galvanometer circuit is closed when the battery key K f is closed, a moment- 

 ary pulse of current may flow through the galvanometer even if equation (82) is satis- 

 fied. In order that this momentary pulse ofRjrrent may not flow, a certain relation must 

 exist between the inductances of the four arms of the bridge a, (3, y and d. This 

 pulse of current due to inductance is made use of in Maxwell's method of measuring 

 inductance by means of the Wheatstone's bridge. See Practical Physics, Franklin, 

 Crawford and MacNutt, Vol. II, pages 129-133. 



