with the generator, unless overloaded, when it will stop. Such a motor is called a synchronous motor. Synchronous motors have the disadvantage of requiring an auxiliary starting-motor. They are seldom used, except in very large plants where the load is required continuously. There are several long-distance transmission-plants in the Rocky Mountains using synchronous motors. All practical self-starting A. C. motors are induction-motors. This motor is based on a discovery of Professor Ferraris of Turin University, Italy, but the first commercial induction-motors were due to Tesla. The armature of an induction-motor is a closed coil without commutator, collector-ring, brushes or external circuit. The multipolar field-magnets are excited by two or more alternating currents which differ in phase. By proper windings and connections of the coils of the field-magnets, there is thus produced a magnetic field which changes both strength and direction, a so-called rotary field. This induces currents in the armature-coils, and carries the armature with it. Induction-motors are self-starting, are simple in mechanical construction, require little attention in operation and are efficient; but they require two or more separate currents to operate them. These currents, however, are generated by a single dynamo. Induction-motors at present are not adapted for purposes where the speed has to be varied, as on street-cars. For small motors, such as fan-motors, the two phases can be obtained by splitting a single-phase current, but this method has not been successfully used on large motors. Both two and three phases are used in A. C. motors.
A DIRECT-CURRENT MOTOR
ARMATURE OF AN INDUCTION-MOTOR
The first persons to make practical use of electric motors were Jacobi, Davenport and Page. Jacobi ran a boat carrying 14 passengers at St. Petersburg in 1838. Davenport had an experimental electric railroad at Springfield, Mass., in 1835. Page ran a 16 H. P. electric locomotive at a speed of 19 miles per hour on the railroad between Washington and Baltimore in 1851. All obtained their electric currents from primary batteries. The first case of running a motor from a dynamo was that of Fontaine and Bregnet, at the Vienna exposition in 1873. The distance between the generator and motor was over a mile. Electric motors are now used in every place where power is needed. The fact that in many shops electric motors are placed on each machine, the motor-efficiency being higher than that of shafts and belting, speaks for the efficiency of the modern electric motor. A reliable engineer in the Engineering Magazine estimates the aggregate power of the electric motors in use as not less than one and a quarter-million horse-power. (See, further, Transmission of Power and Electric Railway.)
.
Electric Railway, a railway using electric motors to move its cars. The first electric railway was a small line built by Thomas Davenport, at Springfield, Mass., in 1835. In 1851 Prof. C. G. Page ran a 16 H. P. electric locomotive at the rate of 19 miles per hour near Washington. The currents in the above cases were from primary batteries, too costly a source for commercial purposes. In 1879 Werner Siemens ran an electric railway 900 feet long at an exhibition held in Berlin; and two years later a line, 1½ miles long, was opened in a Berlin suburb. The currents were generated by dynamos. During the next few years various experiments were made with varying success. The first line which can be said to have been an engineer-
