24 G 
DR. J. HOPKINSON AND MR. E. WILSON ON 
current has the effect of retarding the current (x) with regard to the potential 
difference, and consequently for the same watts transmitted and the same potential 
difference the current must he increased. The case in which the conductors between 
motor and generator have considerable induction or capacity has not been worked 
out. 
The losses in the system can be supplied electrically (instead of by belt as in these 
experiments) as in the case of direct current machines. # 
III. 
The results of the last section are valuable in relation to the effects of induced 
currents in the magnets, the subject of Section I. With a machine working as a 
simple generator the current lags behind the electromotive force on open circuit by 
any amount from 0° to 90°. But when a generator and motor are run rigidly coupled 
together, the current may lead the generator electromotive force, or may lag and the 
motor may lag by any amount from 90° to 270°. Regarding tire relative phases of 
electromotive force of machines and of current, the machine is a generator when the 
current is from 0°to 90° behind the machine ; it is a motor from 90° to 270°, and again 
a generator from 270° to 360°. We have already stated that we should naturally 
expect that the induced currents in the magnets would have little or no effect when 
E.M.F. and current were in the same phase, and that they would have a maximum 
effect when the two were 90° apart, or at quarter centres. We should expect further 
that, as a generator can be made into a motor by reversing the current in the 
armature, wherever local currents diminish E.M.F. of a generator, they would 
increase E.M.F. of a motor. That is, we should expect that local currents would 
diminish E.M.F. from lead 0° to 180°, and increase E.M.F. from 180° to 360°. As a 
fact, we find this to be partially verified ; it seems that local currents diminish 
E.M.F. from a negative angle of comparative small amount, perhaps 30°, to consider¬ 
ably more than 90°, and that they increase E.M.F. from 180° to over 270°. 
Referring to the curves in fig. 14, x and E Cl are in phase, and Je G dt would need 
increasing 3 per cent, to meet JE G dt, when x vanishes E M lags 216°, and Je M dt needs 
diminishing, that is the currents have increased the E.M.F. fn fig. 15 a very small 
change in the observations would change the character of the results. 
We have taken another set of curves shown in figs. 19 and 20. In fig. 19, we 
have a very small current 3'3 amperes in the generator magnets, and the current is 
in phase with the generator, the motor being 264° behind the current. The generator 
is about 12 per cent, low owing to local currents, and the motor is 25 per cent, of 
its actual value high. To obtain a better standard rve excited a machine with 
3'3 amperes and passed the same current as before through the armature and an 
ordinary non-mductive resistance, and found the current lagged 72°, and the E.M.F. 
* See ‘ Engineering,’ 24th March, 1893. 
