26 



ELECTROMOTIVE FORCE AND CURRENT. 



6?i, G.,. Two Alternators coupled mechanically. 



F^ F.,. Alternator field windings. 



FI, F. Voltmeters for reading voltage of each machine 



separately. 

 V 3 . Voltmeter for reading resultant voltage. 



R lt JR 2 . Field regulating resistances. 

 S. Switch in field circuit. 



Note. In the case of this and a number of other experi- 

 ments it is not necessary to use the full number of voltmeters 

 shown in the diagram. A single voltmeter with throw-over 

 switch may be used to take the readings of the two alter- 

 nators. This is often preferable, not only in order to save 

 instruments, but also in order to save errors due to uncertain 

 constants of the instruments, &c. 



Instructions. Two alternators have their shafts mechani- 

 cally coupled together by a coupling which can be adjusted 

 to alter the relative angle of the rotating parts of the machines. 



Excite the field windings from a supply of continuous 

 current. 



Insert in the field of each machine a field regulator, so 

 that the fields can be independently regulated. Connect a 

 voltmeter to the terminals of each alternator. Connect the 

 two armatures together, putting a third voltmeter in this 

 circuit, in order to read the resultant voltage of the two 

 alternators. 



Run the alternators at a constant speed, and take 

 simultaneous readings on the three voltmeters, entering the 

 readings in the proper columns of the table given below. 

 Make the same observations for a series of relative positions 

 of the machine coupling, which should be altered from the 

 point where the machines are in series to the position when 

 they are in parallel, or opposition. The readings should be 

 entered as shown in the form below. 



From the readings of the voltmeters the angle of phase 

 difference between the machines can be calculated, without 

 reference to any graduation of the coupling. 



