142 
Proceedings of the Royal Society 
poles, both of the field magnets and of the armature, are simply 
interchanged, and the rotation is again in the direction opposite to 
that in which it runs as a generator.* It thus appears that when 
a current of electricity is passed through an ordinary dynamo 
machine, the armature always rotates in one direction, whatever be 
the direction of the current. If, however, the field magnets or the 
armature be furnished with a current fixed in direction, the rotation 
of the armature may be reversed by reversing the direction of the 
current in the other part of the machine. 
Such being the case, it would seem likely that if currents of 
varying direction be passed through a dynamo they would cause 
continuous rotation in one direction. I sent the current from a 
Siemens alternate-current machine through a small Griscom motor, 
which has a Siemens armature with only two reversals of the 
commutator in each revolution. Under these conditions the motor 
did not move at all, in whatever position of the armature it was 
tried. The reason of this is that the reversals of direction of the 
current are too rapid to allow of the complete reversal of the mag- 
netism, both in the field magnets and the armature. This has been 
noticed before, and consequently it has been supposed that motive 
power cannot be transmitted by an alternate-current machine. 
When the apparatus was in the same condition as described 
above, I gave a rapid rotation to the motor by hand, and a,fter it 
had reached a certain speed the alternate-current maintained the 
rotation at a steady and uniform speed. I put some pressure on 
the spindle of the motor. It rotated at the same speed. I increased 
this pressure. The speed did not vary. But on continuing this 
action I arrived at a point where suddenly the driving action ceased, 
and if the motor continued to turn it was almost entirely owing to 
its own momentum ; but not entirely so, for on removing the 
pressure entirely, while the motor was still running slowly, the 
speed rapidly increased until it reached the previous uniform rate. 
The sudden diminution of driving power, when a certain amount of 
friction was applied, was very remarkable. 
What is the explanation of this action ? The fact that a uniform 
speed of rotation is maintained, and never exceeded, points to a 
synchronism between the revolutions of the armature and the 
* I only speak of the magnetic actions for simplicity of expression. The 
electric actions are in the same direction as the magnetic. 
