Electro-Measuring Instruments. 309 



iron pole-pieces combined with the soft-iron bridge enables us 

 to use much more powerful controlling magnets, and therefore 

 to obtain a much greater dead-beatness, and a greater freedom 

 from disturbance produced by extraneous powerful magnets, 

 with our last form of voltmeters than with our earliest. 



German- Silver or Copper Wire for the Coil of a Voltmeter. — 

 It might appear at first sight that German-silver wire would 

 be better than copper wire for voltmeters. As regards the 

 error arising from the change of temperature of the room this 

 is undoubtedly the case, but not as regards the heating of the 

 coils by the passage of the current. For, to produce any 

 given deflection on a voltmeter provided with a given control- 

 ling permanent magnet requires a given current for a given 

 number of convolutions ; hence, since the specific resistance 

 of German silver is about 13 times that of copper, the produc- 

 tion of heat, which necessarily accompanies the production of 

 a given magnetic effect with a given instrument, will be 13 

 times as great if German silver be used than if copper be used. 

 And on account of the inferior heat-conducting power of the 

 German silver the elevation of temperature will be probably 

 more than 13 times as great. But for the same elevation of 

 temperature the increase of resistance of copper is only 8*8 

 times the increase of resistance of German silver. Consequently 

 copper wire will be decidedly better than German-silver in any 

 case where the change of resistance due to the heating pro- 

 duced by a current flowing through a given coil is concerned. 



The question whether all the wire should be wound on the 

 bobbin of the voltmeter, or whether a portion only should be 

 on the bobbin, and the remainder of the resistance, necessary 

 to be employed to prevent the voltmeter shunting too much 

 of the main current, should be in the form of a resistance-coil, 

 must be answered by the consideration as to whether the total 

 consumption of energy which takes place when a particular 

 deflection is maintained in the voltmeter, or the variation, 

 arising from the heating of the bobbin, of the reading for a 

 fixed potential difference, is the more important. 



If it is desired that a particular deflection of the needle shall 

 be maintained with the least total consumption of energy, then 

 all the wire should be wound on the bobbin of the voltmeter ; 

 whereas if the heating-error is to be reduced to a minimum, 

 then a large portion of the total resistance in the voltmeter 

 circuit should be external to the voltmeter itself, and should 

 be in the form of a resistance-coil made of German-silver 

 wire, of sufficient thickness that its resistance is not sensibly 

 increased when the current necessary to deflect the pointer of 

 the voltmeter to the limit of its scale is maintained. 



