MEASUREMENT OF POTENTIAL DIFFERENCE 237 



springs need not be kept as low as in ammeters. For direct- 

 current work, the moving-coil type of instrument has become the 

 standard; for alternating currents, the electrodynamometer 

 type is usual, though there are some soft iron and induction 

 instruments. 



In direct-current voltmeters the resistance is about 100 ohms 

 per volt of full scale reading; the resistance of alternating-current 

 voltmeters of the electrodynamometer type is much lower, about 

 20 ohms per volt. It is customary to make the final adjustment 

 of the instruments by altering the series resistance, but it is more 

 convenient, when multipliers are to be used, to have a definite 

 resistance per volt and to effect the adjustment in some other 

 manner. 



In direct-current portable instruments having ranges up to 

 about 750 volts, it is usual to place the series resistance within 

 the base. Self-contained alternating-current portable voltmeters 

 having a range of 300 volts may be obtained. 



When using a voltmeter, it should be kept in mind that it only 

 shows the P.D. between its own terminals and that this is not 

 necessarily the same as the P.D. which previously existed between 

 the points on the circuit to which the terminals are applied. 

 For example, suppose there is a large resistance, 32,000 ohms, 

 across which t he drop is 200 volts and that it is desired to measure 

 the P.D. between one terminal and a tap at the middle of the 

 resistance. Obviously, the P.D. in question is 100 volts; however, 

 if [i voltmeter of 16,000 ohms resistance is applied between one 

 terminal and the tap, it will read 66.6 volts. The application 

 of the voltmeter has changed the quantity which it is desired to 

 measure by 33 per cent. The disturbance of the circuit condi- 

 tions diminishes as the resistance of the voltmeter is increased 

 and would be nil with an instrument which operated on open 

 circuit, that is, an electrostatic voltmeter. In engineering work 

 this difficulty is not often met but one should not lose sight of 

 the possibility. 



Effect of Temperature. It is evident that a high resistance, 

 which must not be subject to changes due to the heating action 

 of the current or to alterations of room temperature, is an 

 essential part of any electromagnetic voltmeter. As the 

 instrumental errors should be practically independent of tern- 



