Potentiometer on Alternate Current Circuits. 



4(0 



The readings were obtained with great ease and defmiteness, 

 half a division on the slide-wire, or "0005 volt, being easily 

 detected, in spite o£ the fact that trouble was found from 

 mechanical vibration, as the alternator was in the same room. 

 There seems no reason to doubt that *0002 volt could be 

 easily detected, or that a drop of *1 volt could be measured 

 to an accuracy of '2 per cent, or closer. The readings of 

 the phase angle repeated themselves constantly to within 

 *1 deoree. 



To give some idea of the accuracy of phase measurements, 

 a resistance and ironless choking-coil were connected in 

 series w T ith the *04 co resistance, and an alternating current 

 of 15 amps, at GO ~ passed through the combination. 

 Readings were then taken of the P.D. across the '04 o> 

 resistance, across the choking-coil, total resistance, and the 

 whole circuit, the last three being taken by the aid of a 

 10 x 1 volt box. The following are the results : — 



Circuit. 



First Test. 



Second Test. 



P.D. 



•5960 

 3-117 



4-880 

 6-185 



Phase-angle, 

 degrees. 



46-5 



47-1 



125-9 



953 



P.D. 



Phase- 

 angle. 



•04 resistance ... 

 Total resistance . 

 Choking-coil 



3-108 

 4-928 

 6218 



473 

 125 

 949 





The difference between the first and second readings is due 

 to the fact that a carbon resistance was employed which 

 became heated. 



Fig. 4 (p. 410) is a graphical representation of the result in 

 the last case, the horizontal line representing the resistance 

 drop, and the other two lines the drop across the choking-coil 

 and wdiole circuit respectively. Although the triangle does 

 not quite close, the result may be taken a- satisfactory in con- 

 sideration of the fact that single phase supply was used in 

 this case, and the phase was split by condensers. It should 

 be noted also that errors in the phase angle do not in any 

 way affect the accuracy of the magnitudes of the P.Ds. 



the writer has designed a potentiometer in which a small 

 phase-shifting transformer and dynamometer are included in 



