ELECTRICITY AT THE WORLD'S FAIR. 735 



sion, because it has lacked the prime requisite for such a use a 

 satisfactory motor. This missing link in the chain of appliances 

 necessary to render the system complete has in recent years been 

 supplied by the discoveries and inventions of Mr. Nikola Tesla, 

 whose remarkable experiments with alternating currents of great 

 tension and enormous frequencies have excited such widespread 

 interest among scientific men. To understand the solution given 

 to the alternating-current motor problem by Mr. Tesla it will be 

 necessary to consider briefly the principle of the electric motor 

 and the cause of the rotation of an armature in a magnetic field. 

 If we take a loop of wire forming a closed circuit and place it be- 

 tween the poles of a magnet it will tend, when a current is flow- 

 ing through it, to set itself so as to inclose the greatest number of 

 lines of force that is, in a plane at right angles to the line joining 

 the magnetic poles. If the mechanical inertia of the moving loop 

 carry it slightly past its position of equilibrium, and at the same 

 moment the current through the loop be re versed, it will be pulled 

 around by the attraction of the magnetic poles to a new position 

 of equilibrium ; and if at each of these positions there takes place 

 a reversal of the current, continuous rotation of the loop will be 

 produced. Where there are many loops, as in actual machines, 

 the pull upon the moving system of coils tending to rotate it will 

 be continuous and equal at all points of the rotation, as, while 

 some coils are approaching and passing through the position of 

 equilibrium, others are in position to have exerted upon them the 

 maximum strain. The pull of the field magnets upon the moving 

 conductors is greatly increased if these be wound over an iron 

 center, as in this case each loop tends to set up magnetic poles in 

 this core in a position at right angles to its plane. Two magnetic 

 poles attract each other when of different polarity and repel each 

 other when of the same polarity. The poles of the iron core are 

 consequently repelled and attracted by the field poles with each 

 change of the direction of the current, and this occurs in exact 

 synchronism with the changing forces acting upon the wire cir- 

 cuits. It must, of course, be understood that with a continuous 

 current the direction of the current in space is always the same. 

 The alternating current impulses set up in the armature coils of 

 the direct-current dynamo are through the device of the commu- 

 tator made to follow each other in the same direction through the 

 line. Arriving at the motor, these impulses pursue a continuous 

 course through the armature always in the same direction, the 

 positive current always flowing in by one brush and the negative 

 out by the other. The armature coils, however, by reason of their 

 rotation, present their two ends in succession to the positive and 

 negative brushes, and hence are alternately traversed by the cur- 

 rent in reverse directions. If now the commutator be suppressed 



