240 
DR. J. HOPKINSON AND MR. E. WILSON ON 
be greatly diminished by compelling the magnetizing current to be constant in the 
magnet coils, nor can it be greatly increased by exaggerating the currents induced 
about the magnets by intentionally introducing additional conductivities around 
them. The effect of each is merely to alter the place where the currents occur. 
Recently machines have been built, with finely-divided pole pieces to the magnets 
by Messrs. Mather and Platt and by the British Thomson-Houston Company. 
It was obviously desirable to obtain a verification with a machine of totally 
different construction. For this purpose we had available the first model made of 
the alternating machines manufactured by Messrs. Mather and Platt. It has an 
iron core in the armature which projects and extends beyond the armature coils. It 
was treated in exactly the same way as the Siemens’ machines but with a fairly full 
load. The results are shown in fig. 21, from which it will be observed that the total 
induction actually observed when the machine is loaded is about 11 per cent, below 
the induction inferred from the electromotive force on open circuit. 
II. 
The following experiments were primarily made for the purpose of determining 
the efficiency of the machine, but they will be seen in Section III. to have an 
important bearing upon the principal subject of this paper. 
For the purpose of finding the efficiency of the alternators, when running as 
generator and motor, # the two armatures were rigidly mechanically coupled together, 
* This is the same method of test which has been applied to direct current machines (see ' Phil. 
Trans.,’ R.S., 1886, p. 331). 
