for Alternating Currents. 203 



a current in the resistance b c, which is regulated by an 

 adjustable resistance so that the standard cell e is balanced 

 when its contact/ is touched down at the mark corresponding 

 to its electromotive force at the temperature at which it 

 stands ; k is an alternating dynamo, and h is the alternating 

 voltmeter to be calibrated. The differential electrometer is 

 connected up as shown. The double fishtail-shaped needle 

 is pulled one way by a force varying as the square of the 

 direct electromotive force, and the other way by a force vary- 

 ing as the square of the alternating pressure. The fishtail 

 shape is necessary to ensure that the needle is in stable equi- 

 librium when the forces are equal. Fig. 1 is, of course, a 

 mere diagram. Resistance-coils for adjustment are left out, 

 and b c is shown as a stretched wire, whereas it is made up of 

 resistance-coils. As to sensitiveness, it depends on the con- 

 struction of the electrometer ; but, as already explained, there 

 is no controlling force except that due to the suspending fibre. 

 Take as an example an instrument that will indicate one volt 

 when there is no pressure on the other side : it will admit of 

 the comparison of two pressures of approximately 100 volts 

 within one in ten thousand ; and it will serve for comparing 

 2000 volts within one in four million. To find the true zero 

 of the instrument the four quadrants are connected together. 

 The key for making this connexion is not shown. 



One of the great advantages of the potentiometer method 

 in direct-current work is the ease with which large currents 

 can be measured by the fall of potential over very small 

 resistances. As the electrometer must be used idiostatically 

 in alternating work it is not sensitive to very small pressures; 

 and the method given is, unfortunately, useful only for 

 pressure measurements. Accurate current measurements can 

 be taken by the use of a differential dynamometer. This 

 instrument has two fixed and one moving coil. The direct 

 circuit is through one fixed and the alternating through the 

 other fixed coil, and both circuits are led through the moving 

 coil. The controlling spring is removed. This method is 

 somewhat more complicated than in the case of pressure, and 

 mercury contacts are necessary. The arrangement is shown 

 in fig. 2. The battery, a, supplies a direct current, which is 

 measured by the fall of potential over the low resistance, b, 

 and which passes in one of the fixed coils, c, and the moving 

 coil, d. The alternating-current circuit is led through the 

 circuit, /, whose current is to be measured, and through the 

 other fixed coil, e, and the moving coil, d. There is a slight 

 error due to the whole of the alternating circuit being in 

 shunt to the moving coil considered as part of the direct- 

 current circuit, and vice versa ; but this is very minute. If 



