406 Dr. C. V. Drysdale on the Use of the 



direct current working, except that a vibration galvanometer 

 is substituted for the ordinary form. By having an auxiliary 

 battery B and throw-over switch S, the arrangement is 

 always ready for either direct-current or alternate-current 

 comparisons. 



The procedure is then as follows : — The throw- over switch 

 is first arranged /to supply direct current from the battery B T 

 and the rheostat altered until the standard cell is balanced at 

 its nominal value, as in ordinary direct-current working. 

 The dynamometer reading is then carefully noted, or a 

 fiducial mark is made, which always represents the exact 

 *05 ampere. On now throwing over to alternate current, the 

 rheostat is altered to reproduce the same R.M.S. current as 

 indicated by the dynamometer D ; and the position of the 

 potentiometer contact is shifted and the phase-shifter turned, 

 until the vibration galvanometer shows no vibration. The 

 adjustment is very similar to that of the two resistances in an 

 Anderson's bridge, and presents no difficulties. The voltage 

 and phase are then read off on the potentiometer and phase- 

 shifter respectively. In the illustration a low resistance is 

 connected in series with the load, and a volt-box in parallel 

 with it, and the current and PJ). can be determined directly 

 in the ordinary way. Of course the phase-shifting trans- 

 former must always be connected to the same source of 

 supply as the load. 



Instead of the vibration galvanometer, a dynamometer or 

 electrometer in which one pair of terminals is connected to 

 the supply so as to be " separately excited." and the other 

 to the ordinary galvanometer terminals, has been employed 

 with good results. The deflexions are then to left or to 

 riodit as in an ordinary galvanometer. But it must not be 

 forgotten in this case that balance will be obtained not only 

 when the vectors of the two P.Ds. compared are coincident, 

 but also when their vector difference is in quadrature with 

 the P.D. of the supply. For this purpose, if such an instru- 

 ment is used it must have its " exciting terminals'''' changed 

 over from one phase to the other, and balance secured in 

 both cases. 



Before giving examples of readings obtained in this 

 manner, we must consider what is really measured. It is 

 obvious that this device does not indicate the effective or 

 B.M.S. value of the P.D., except when the supply and tested 

 wave-forms are both sinusoidal or of identical form. When 

 a vibration galvanometer is employed, its sensitiveness to the 

 fundamental wave is so great in comparison with that to 

 the harmonics, that we shall be practically correct in assuming 



