7 2 



ALTERNATING CURRENTS 



of being practically non-inductive and unaffected by external fields, 

 has recently been devised by Dr. C. V. Drysdale, and is shown 

 in Figs. 52 and 53. The general arrangement of the working 

 parts will best be understood by reference to Fig. 52. The main 

 or series coils are wound on laminated cores which form a nearly 

 closed magnetic circuit, there being only two small air-gaps. In 

 these air-gaps are arranged the two shunt coils, one of which is 

 fixed against the ends of the right-hand core, while the other is sup- 

 ported by springs in the manner shown, and possesses freedom of 

 motion up and down. The current is arranged to circulate in oppo- 

 site directions around the shunt coils, so that their self-inductance 

 is practically wiped out. Further, by reason of the very intense 

 field due to the series coils, the instrument is not liable to be 

 disturbed by external fields, and a still higher degree of security in 

 this respect is obtained by enclosing the instrument in a cast-iron 

 case. Fig. 53 shows the actual instrument with the front cover and 

 a portion of the scale removed so as to exhibit the mechanism of the 

 instrument. 



37. Power Factor Indicators 



The power factor ( 7) can always be calculated from the readings 

 of a voltmeter, ammeter, and wattmeter. Instruments have, how- 

 ever, been devised for indicating directly the value of the power 

 factor. Such instruments are known as 'power factor indicators. In 



Fig. 54 is illustrated the general 

 arrangement of a type of power 

 factor indicator devised by Punga, 

 and manufactured by Messrs. 

 Everett, Edgcumbe & Co. It is 

 intended for use on a three- 

 phase circuit, and consists essen- 

 tially of two systems of coils, 

 a fixed system of current coils 

 and a movable system of pressure 

 coils, the coils of each system 

 forming angles of 120 with each 

 other. Currents are led into 

 and out of the movable system 

 by means of very weak strips of 

 phosphor-bronze, which exert no appreciable controlling couple. 

 Each system gives rise to a rotating magnetic field when traversed 

 by three-phase currents ( 15). So long as the two fields coincide, 

 no torque will be exerted on the movable system. But any change 

 in the relative phase of the currents will cause a displacement of the 



FIG. 54. Power Factor Indicator. 



