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VI. Alternate Current Dynamo-Electric Machines. 
By J. Hopkinson, F.B.S., and E. Wilson. # 
Received April 4,—Read May 2, 1895. 
The paper deals experimentally with the current induced in the coils and in the 
cores of the magnets of alternate current machines by the varying currents and by 
the varying positions of the armature. It is shown that such currents exist and that 
they have the effect of diminishing to a certain extent the electromotive force of the 
machine when it is working on resistances as a generator without having a corres¬ 
ponding effect upon the phase of the armature current. It is also shown that 
preventing variations in the coils of the electromagnet does not, in the machine 
experimented upon, greatly affect the result, and that the effect of introducing 
copper plates between the magnets and the armature has not a very great effect 
upon the electromotive force of the armature, the conclusion being that the con¬ 
ductivity of the iron cores is sufficient to produce the main part of the effect. A 
method of determining the efficiency of alternate current machines is illustrated 
and the results of the experiments for this determination are utilised to show that 
in certain cases of relation of phase of current to phase of electromotive force the 
effect of the local currents in the iron cores is to increase instead of to diminish the 
electromotive force of the machine. 
I. 
In algebraic discussions of the theory of alternate current machines, it has usually 
been assumed that the electromotive force due to the magnets is a periodic function, 
the same whether there is a current in the armature or not, and that the effect of the 
current in the armature can be represented by regarding the armature as having 
self-induction. It has been pointed out, too, that the coefficient of self-induction 
will generally vary with the position of the armature in the field. To state exactly 
the same thing in another way it has been assumed that the electromotive force of 
the magnets is a periodic function independent of the current in the armature, and 
that the effect of the armature current on the induction through the armature can be 
* The large majority of the experiments herein described were made in the summer of L893 and a 
considerable part of the paper was then written. We have to thank Mr. F. Lydall, one of the student 
demonstrators at King’s College at that time, for much assistance, 
6.5.96, 
