240 



ELECTRICAL MEASUREMENTS 



sistance as is common in direct-current instruments. This is a 

 disadvantage, for it increases the liability of altering the circuit 

 conditions by the application of the instrument. The resistance 

 of a 150-volt instrument of this type is from 2,500 to 3,000 ohms. 

 Low ranges are obtained by reducing the series resistance, and 



as the inductance remains the 

 same, the likelihood of a fre- 

 quency error is much increased. 

 In investigation work it is 

 sometimes necessary to measure 

 voltages on circuits of abnor- 

 mally high frequency, 500 to 

 1,000 cycles per second. In this 

 case, if a dynamometer volt- 

 meter is used, the inductance 

 term will not be negligible. Its 

 value will depend on the posi- 

 tion of the movable coil. Eddy 

 currents in the metal frames as 

 well as capacity effects between 

 the coils and in the series resis- 

 tance also modify the action of 

 the instrument. 



FIG. 131. Shielded dynamometer voltmeter, General Electric Co. 



A certain alternating-current voltmeter of the electrody- 

 namometer type had a resistance, when measured with direct 

 current, of 1,557 ohms. This agreed with the resistance meas- 

 ured with an alternating current of 16 cycles per second. The 

 effective inductance at 16 cycles per second (and 110 volts 

 deflection) was 0.061 henry. At 3,000 cycles per second the 

 effective resistance was 1,675 ohms while the effective inductance 



