136 Professor C. A. Cams Wilson [April 28, 



figure. If the spot moved quickly enough, the successive impressions 

 would remain on the eye for a sufficient length of time to give the 

 effect of a continuous line of light. 



When examined in this way, the force due to a constant magnetic 

 field and a variable current in the strip was seen to vary from nothing 

 to a certain maximum, then to nothing again, then to change sign 

 and increase to an equal negative maximum, and finally to fall to 

 nothing. The strip was thus subjected to a series of equal impulses 

 varying in sign so that the resultant impulse was zero. 



To prevent the impulses from changing sign it was necessary to 

 make the magnetism change sign at the same time as the current. 

 This could be done by exciting the magnet by an alternating current 

 in step with that in the strip. When this change was made, the spot 

 of light was seen to fluctuate up and down from a fixed line, instead 

 of alternating as before, showing that the strip was now subject to a 

 series of fluctuating impulses. There were, however, dead-points at 

 which the strip was not subject to any force. It appeared then 

 possible to combine an alternating field with a conductor or con- 

 ductors carrying an alternating current in such a way as to obtain a 

 series of unidirectional impulses, and motors had been constructed on 

 this principle. Such motors had the disadvantage of dead-points, 

 and uniform motion could be obtained only by making the moving 

 parts of considerable weight, so that part of each impulse was stored 

 up as kinetic energy, and the dead-points thus successfully passed 

 over. 



The dead-points could be avoided in a manner often used in 

 steam engines, where two separate cylinders were set so that when 

 the action of one on the crank shaft was a minimum, the action of 

 the other was a maximum. This principle could be carried out far 

 more completely in an electric motor than in any other form of 

 motor for the reason that while in a steam engine, for instance, the 

 force of each one of the two cylinders varied according to an har- 

 monic law, with uniform steam pressure, and two series of harmonic 

 impulses at right angles did not give a uniform turning moment, 

 with the electric motor, on the other hand, the law of force variation 

 with synchronously varying field and current was not an harmonic 

 law — as might be seen by the shape of the curves on the screen — 

 but a law having the remarkable peculiarity that two such series of 

 impulses at right angles gave a uniform impulse when acting 

 together. It was thus possible to construct an alternating electric 

 motor in which the sum of the turning moments on the shaft was a 

 constant quantity. The Induction Motor was such a motor. 



Taking the fixed coil A, shown in plan in the figure, to represent 

 the magnetising coil used in former experiments, and the moving coil 

 B to represent the conductor or strip, the condition of a unidirectional 

 fluctuating impulse from A to B was that the magnetism of A should 

 be in step with the current in B. To produce the latter recourse 

 might be had to the principle of induction, by which currents could 



