250 Mr. B. Hopkinson. [June 16. 



(2) and (5) above. It is worth while in this case, the conditions of 

 which approximate to those assumed in the theory, to determine the 

 order of magnitude of y and y'. 



The following measurements were made : 



Total reactance (L^>) varied between 2 '81 and 3'18 ohms, according 

 to position of armature. Its value may. be taken as constant, and 

 equal to 3 ohms without serious error. 



Total resistance (p) = 1*2 ohms. 



In case (c) the potential across the motor terminals (that is excluding 

 the external self-induction) was 75 volts. Hence, Ap is less than 

 75 .^(2) volts. On the other hand, Ap is greater than ^/(2) times the 

 open-circuit potential of the machine with exciting current 30 '5, which 

 was measured and found to be 65 volts/ Take, therefore Ap as equal 

 to 70 ^/(2). The power supplied to motor terminals (again excluding 

 the external self-induction) was 1000 watts. The current was 

 13*5 amperes, hence the C 2 R loss in armature is 110 watts (armature 

 resistance taken as 0'6 ohm), and we have 



= 890 watts, 

 and 



ft, = 12-7 v /(2) amperes. 

 Thus we obtain 



(See formula 11 above.) 



For y the more accurate formula (9) must be taken, since, as in all 

 experiments with this motor, the resistance term is important. The 

 formula may be written with sufficient accuracy 



We take ja = ^ sec., 2?r/8 (the period of oscillation) = 0*36 second, 

 - = Jg- second Hence Sis 17 and p = 314. 



We have 

 Hence 



App 70V(2)xl-2 ,,_, . 



^4; - = 8 v/( 2 ) amperes. 



10-4 



_ 0-5xO-lx3URl2-7)M8) 2 ]x2 

 (M x (17'-' + l-3 



This value is, of course, very rough ; the most that can be asserted 

 is that y' is a positive quantity of the same order of magnitude as, and 



