ELECTROMOTIVE FORCE AND CURRENT. 



29 



The preceding experiment is not generally capable of any 

 direct useful application, and is given here chiefly as illustrat- 

 ing the way in which two voltages may be applied to a circuit 

 and the phase angle between them obtained by direct measure- 

 ment of the resultant. 



It should be noted, however, that two alternators which 

 can be coupled at any angle form a valuable addition to the 

 equipment of a test-house where meters, &c., have to be 

 tested with currents of varying phase difference. In such 

 cases current may be taken from one machine and voltage 

 from the other, with any desired angle between them. It 

 might, therefore, easily occur that this experiment might 

 be turned to practical purposes. 



The geometrical construction made use of in the last 

 experiment is employed in the following measurement, 

 which has many important applications. 



EXPERIMENT II. DETERMINATION OF ANGLE OF LAG IN 

 AN ALTERNATING CIRCUIT BY THREE VOLTMETERS. 



DIAGRAM OF CONNECTIONS. 



FIG. 13. 



. '. e\ + e 2 = E [sin + sin (0 + <p)] = 2 E sin (# + -^-) cos 



Where 9 = phase angle of machine I. 

 ?> = angle between machines. 



(D 

 Hence voltmeter reading = 2 cos -^- x resultant voltage. 



R.M.S. value of sin (0 + -|- ) = 



1 



.: R.M.S. value of 



The meaning of " R.M.S." value is explained later. 



+ E> 2 } = 2E cos 



