ALTERNATE CURRENT DYNAMO-ELECTRIC MACHINES. 
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the square of the current, so that we may expect the two effects, that due to the 
reaction of the armature on itself and that brought about by the armature inducing 
currents in the magnets, to vary together. 
The lag of phase is less than we should expect from the diminution of electromotive 
force, or the electromotive force suffers greater diminution than we should expect 
from the angle of lag of phase. 
We have tried a number of experiments for the purpose of tracing the variations 
of current in the magnets, and also with the intention of increasing or diminishing 
the effect we have observed. It is easy enough to trace the variations in the current 
round the magnets by measuring at points during the period the potential difference 
between the ends of a non-inductive resistance in series with the magnets. These 
variations are shown in figs. 6 and 7, in which the armature bobbins were coupled, 
4 series, 3 parallel, and 12 series, respectively. We see, as we should expect, that 
the variations have a periodic time one-half the periodic time of the machine. But 
the current round the magnet could be made constant by exciting the two machines 
with the same current, loading each to the same degree, and placing their armatures 
one-fourth part of a period apart in phase. 
The machines were run under conditions set forth in Table I., and the curves are 
given in figs. 5, 6, 7, 8. An electromotive force curve E, was observed with no 
current in the armature, and a curve of potential difference was taken between the 
brushes with the alternator loaded on a non-inductive resistance. Rx is this curve 
with variations in the exciting current, and Rx' without variations, in each case 
corrected for the resistance of the armature. Figs. G and 7 show the curves of actual 
electromotive force, Rx and Rx', when the current in the magnet winding is allowed 
to vary and when its variations are stopped. It will be seen that they do not differ 
a great deal. What currents are stopped in the magnet winding no doubt turn up in 
the substance of the cores themselves and have an effect not differing greatly. Curve E 
represents electromotive forces observed. Rx represents the potential difference taken 
between the brushes and corrected for the resistance of the armature, with the 
alternator working on a non-inductive resistance with variations in the exciting current. 
Rx' ivithout such variations in each case. The induced currents are in either case 
adequate to nearly stop the variation of induction. Fig. 8 shows the same thing. 
An exploring coil was wound and placed on one of the magnet limbs and the 
electromotive force in it was observed in terms of the time for the various positions of 
the exploring coil, marked 1, 2, 3, 4, 5, in fig. 1. Both the amplitude and the epoch 
varied with the position of the coil, but, in all cases, the periodic time was half the 
periodic time of the machine. It does not seem worth while to publish the curves 
connecting electromotive force and time. 
Lastly, we tried to exaggerate the effects ; for this purpose we introduced plates 
of copper, inch thick, in the form of two flat rings between the pole faces and the 
armature. Curves 9, 10, 11, 12, give the results for two different currents round 
