1899.] Some Features of the Electric Induction Motor. 135 



WEEKLY EVENING MEETING, 



Friday, April 28, 1899. 



Sir Edward Frankland, K.C.B. D.C.L. LL.D. F.R.S., 



Vice-President, in the Chair. 



Professor C. A. Carus Wilson, M.A. M.Inst.E.E. 

 Some Features of the Electric Induction Motor. 



The action of a magnetic field upon a conductor carrying an alter- 

 nating current might be illustrated in a simple manner by placing an 

 incandescent lamp in the neighbourhood of an electro-magnet and con- 

 necting the lamp to an alternating supply circuit. If the magnetic 

 field were uniform, the filament would vibrate in front of the magnet, 

 and if the vibrations could be minutely studied they would be found 

 to increase first of all to a maximum in one direction, then fall to 

 nothing, and then reach a maximum in the other direction, following 

 an harmonic law. The variations of the force on the filament were 

 similar to those which took place in a steam engine, where the 

 turning moment on the crank went through a complete cycle from a 

 positive to a negative maximum, the law of variation being harmonic 

 if the steam pressure were constant and the connecting rod of infinite 

 length. 



The conditions under which the conductor vibrated could be 

 shown more perfectly if it were possible to attach a mirror to the 

 conductor and reflect a beam of light therefrom. But in order that 

 the motion of the conductor should give a true indication of the force 

 acting upon it, the moving part must have a very small periodic time 

 and be perfectly dead-beat. These conditions were fulfilled in a 

 remarkable way in the oscillograph recently designed by Mr. W. 

 Duddell, who had kindly lent one of his instruments for the purpose 

 of illustration. In this instrument the conductors were metal strips 

 stretched in a magnetic field ; a minute mirror fixed to the strips 

 reflected a spot of light on to the screen. The delicacy of the 

 arrangement was such that the period of vibration of the mirror was 

 only the two-thousandth part of a second. 



When an alternating current was passed through the strip, the 

 magnetic field being constant, the spot of light on the screen assumed 

 an up and down motion in a straight line. It was possible, however, 

 to indicate the true nature of the law of motion by making the beam 

 of light on its way to the screen strike on a mirror which moved to 

 and fro synchronously with the variations of current in the strip. 

 The spot then followed a wave-like course, tracing out an S-shaped 



