produced rotation by inducing, by means of a single alternating
current passed through a motor circuit, in the mass or other circuits
of the motor, secondary currents, which, jointly with the primary or
inducing current, created a moving field of force. A simple but crude
form of such a motor is obtained by winding upon an iron core a
primary, and close to it a secondary coil, joining the ends of the
latter and placing a freely movable metal disc within the influence of
the field produced by both. The iron core is employed for obvious
reasons, but it is not essential to the operation. To improve the
motor, the iron core is made to encircle the armature. Again to
improve, the secondary coil is made to overlap partly the primary, so
that it cannot free itself from a strong inductive action of the
latter, repel its lines as it may. Once more to improve, the proper
difference of phase is obtained between the primary and secondary
currents by a condenser, self-induction, resistance or equivalent
windings.

I had discovered, however, that rotation is produced by means of a
single coil and core; my explanation of the phenomenon, and leading
thought in trying the experiment, being that there must be a true time
lag in the magnetization of the core. I remember the pleasure I had
when, in the writings of Professor Ayrton, which came later to my
hand, I found the idea of the time lag advocated. Whether there is a
true time lag, or whether the retardation is due to eddy currents
circulating in minute paths, must remain an open question, but the
fact is that a coil wound upon an iron core and traversed by an
alternating current creates a moving field of force, capable of
setting an armature in rotation. It is of some interest, in
conjunction with the historical Arago experiment, to mention that in
lag or phase motors I have produced rotation in the opposite direction