162 Dr. Sumpner on the Measurement 



alternating voltage. By using a commutator in this way, the 

 most delicate Thomson re fleeting-galvanometer can be utilized 

 for measuring alternating voltages of the same magnitude 

 as the smallest direct-current voltages to which such an 

 instrument responds. 



The method would in practice prove very tedious owing to 

 the need for c irefully adjusting the brushes before taking a 

 measurement, since the adjustment required will depend on 

 the phase of the voltage under test. But if the commutator 

 be rotated nearly, but not quite, synchronously with the speed 

 corresponding with the frequency, the galvanometer or 

 voltmeter will show "beats/' the interval between which 

 lengthens as the speed of synchronism is approached. Under 

 such conditions, the phase of the brush contact will auto- 

 matically adjust itself whatever the phase of the voltage 

 under test ; and the maximum reading of the direct-current 

 instrument will measure half the arithmetical mean of the 

 ordinates of the curve representing the alternating voltage. 



Any method of driving in which the speed can be delicately 

 adjusted can be used for running such a commutator. A 

 shunt motor with adjustable resistance either in series or in the 

 shunt circuit would of course do. But for alternating currents 

 of usual frequencies, a particularly convenient piece of 

 apparatus is available in the induction motor. The speed of 

 the rotor of such a motor, when running light, differs from 

 the synchronous speed by a minute amount called the " slip," 

 the amount of which in many cases is less than one-tenth 

 per cent. For currents of 50 cycles per second such a value 

 of the slip would correspond with beats at intervals of 

 20 seconds, or with a 10 second interval between the maximum 

 positive and the maximum negative readings of the direct- 

 current instrument. 



A commutator of very simple construction, as above 

 described, when attached to the spindle of an induction 

 motor, and used with a direct-current instrument, forms in 

 practice a satisfactory means of measuring minute alternate- 

 current voltages. The writer has found it very serviceable 

 in connexion with the measurement of alternating magnetic 

 fields. The reliability of the apparatus was thoroughly 

 tested in various ways, as, for instance, by putting in parallel 

 (1) a standard voltmeter suitable for either direct or alternate 

 voltages, and (2) a direct-current voltmeter in series with 

 the commutator. The ratio of the two readings in volts 

 of the two instruments was found to be exactly 2 on direct- 

 current circuits with the commutator running; but for 

 the alternating voltages produced by a small rotary trans- 

 former run from the direct-current side, the ratio was found 



