154 Mr. Julius Frith on 



could be suddenly withdrawn from between the field coils 

 and the armature, when the latter was at rest with its coils 

 in a line with the field coils. The ends of this coil were 

 connected to a ballistic galvanometer and the kick observed 

 for different values of the magnetizing current on suddenly 

 withdrawing the coil. Curve I. (Plate III.) represents the 

 results of these experiments. 



E.M.F. Curves Varying Speed and Exciting Current. — 

 Curve II. (Plate IV.) shows the E.M.F. curves at the 

 terminals of the dynamo on open circuit, (i) keeping the 

 speed constant and varying the exciting current, and (2) 

 keeping the exciting current constant and varying the 

 speed of the dynamo. 



Current Curves. — Curve III. (Plate V.) shows the effect of 

 taking current from the machine, the resistance coils through 

 which the current passed being nearly without self-induc- 

 tion ; the lag recorded being due to the self-induction of the 

 armature. 



If E, the impressed E.M.F., be of the form 

 E sin//, 

 then in a circuit of self-induction L and resistance R the 

 current is given by 



where Lp 



tana = -~-* 



From the observations shown in Plate V. the equations to 

 the curves are found by the method of least squares to be : 

 For the E.M.F. curve, R = 00 

 E = - 229sin(tf - 2 ) - i6-4sin(20 - 3 ) + 36sin(30 + i°). 



For R = 2i'5, 

 C = - 8'7sin(0 - 20 ) - -i4sin(20 + 84°) + ^sinfofl + 33 ). 



For R = 9, 

 C = - i6'4sin(0 - 31 ) - '4sin(20 - 6i Q ) - 1-68111(38 + 51 ) 



For R=2, 

 C = - 2o'7sin(0 - 64 ) - 7sin(20 - 20 ) - i'9sin(30 + 12 ). 



