DRS. J. AND E. HOPKINSON ON DYNAMO-ELECTRIC MACHINERY. 347 
The following illustrations of the effect of the current in the armature and the lead 
o 
of the brushes are interesting. In both cases the magnet coils are supposed to be 
entirely disconnected so that c is zero. First, let X be negative, short circuit the 
brushes and drive the machine at a certain speed, a large current will be produced, 
the current in the armature itself forming the magnet.* Second, let X be positive, 
cause a current to pass through the armature, the armature will turn in the positive 
direction and will act as a motor capable of doing work. In either case, particularly 
the former, such use of the machine would not be practical owing to violent sparking 
on the commutator. The following is a further illustration of the formula given above. 
If we could put up with the sparking which would ensue, it would be possible to 
make X negative in a generator of electricity, and thereby obtain by the reactions of 
the armature itself all the results usually obtained by compound winding. 
Ffficmicy Experiments. 
Having discussed the relations subsisting between the configuration of the magnetic 
circuit of a dynamo machine and the induction obtained for given magnetising forces, 
and having compared the results obtained by direct calculation with the results of 
actual observation on a particular machine, the construction of which we have described 
at length, it appeared of importance to determine the efficiency of the machine under 
consideration as a converter of energy, when used either as a generator of electricity 
or as a motor. An accurate determination of the mechanical power transmitted to a 
dynamo by a driving belt, or of the power given by a motor presents formidable ex¬ 
perimental difficulties. Moreover, if the mechanical power absorbed in driving the 
dynamo be measured directly, any error in measurement will involve an error of the 
same magnitude in the determination of the efficiency. To avoid this difficulty, we 
employed the following device. 
Let two dynamos, approximately equal in dimensions and power, have their shafts 
coupled by a suitable coupling, which may serve also as a driving pulley; and let the 
electrical connexions between the dynamos be made so that the one drives the other 
as a motor. If the combination be driven by a belt passiug over the coupling pulley, 
the power transmitted by the belt is the waste in the two dynamos and the connexions 
[* Added Aug. 17.—This experiment was tried upon a dynamo machine of construction generally similar 
to that shown in figs. 3, 4, 5, Plate 18, but with an armature of half the length intended in normal work 
to give 400 amperes 50 volts at 1000 revolutions. The magnet coils were disconnected, and the terminals 
of the armature were connected through a Siemens’s electro-dynamometer, and the machine was run at 
1380 revolutions. When the brushes were placed in the normal position (\=0) the current due to 
residua] magnetism was 52 amperes. By giving the brushes a small positive lead the current was 
reduced to nearly zero. By giving the brushes a small negative lead a current of over 234 amperes, the 
maximum measured by the dynamometei’, was obtained, and by varying the lead it was easy to maintain 
a steady current of any desired amount.] 
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