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 between the poles of a magnet it will tend, when a current is flowing 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 reversed, 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 circuits. 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 commutator 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 current in reverse directions. If now the commutator be suppressed
