are always flowing toward one brush b, from which they are led to the external circuit and back at b'. This condition always exists, no matter how fast the rotation; for it will be seen that as each loop rotates into the position where the direction of its current reverses, it passes a brush and therefore at once becomes a part of the circuit on the other half of the drum, where the currents are all flowing in the opposite direction. Fig. 320 shows a typical modern four-pole generator, and in Fig. 322 may be seen more clearly the drumwound armature. Fig. 330 (p. 335) illustrates the method of winding such an armature, each coil beginning on one segment of the commutator and ending on the adjacent segment. 358. Dynamo lighting circuit. The type of circuit generally used in D.C. incandescent lighting is shown in Fig. 321. The lamps are arranged in parallel between the mains.

The field magnets are excited partly by a few series turns which carry the whole current going to the lamps, and partly by a shunt coil consisting of many turns of fine wire (Fig. 321). This combination of series and shunt winding maintains the P.D. across the mains constant for a great range of loads. Such a machine is called a compound-wound dynamo, to distinguish from a series-wound machine, for example, which dispenses with the shunt coil.

Main circuit

S

FIG. 321. The compoundwound dynamo

In all self-exciting machines there is enough residual magnetism left in the iron cores after stopping to start feeble induced currents when the machine is started up again. These currents immediately

EDISON'S FIRST ELECTRIC LOCOMOTIVE AND THE VIRGINIAN ELECTRIC
In 1880 Edison, knowing that his efficient dynamo would also operate as an efficient motor,
placed one of the 12-horse-power machines on its side as shown in the picture and used it at
Menlo Park to draw three cars on a narrow-gauge track one third of a mile long. The Virginian
electric locomotive is the largest in the world. It is used to haul heavy trains over the Allegheny
Mountains. The three parts of the locomotive make a single unit 152 feet long weighing
637.5 tons. The tractive force is 231,000 pounds, with a possible continuous horse power of
6000, reaching a maximum under certain conditions of 7125 H.P., thus exceeding in power any
other locomotive in existence, either electric or steam. On going down grade the necessary braking
action is obtained by allowing gravity to operate the motors as generators, thus turning power
back into the transmission line to help pull other trains which are coming up grade. (Courtesy
of the Westinghouse Electric and Manufacturing Company and of Thomas A. Edison)

This motor-generator set consists of an alternating-current motor (seen at the right end) which may be operated
on 2300 or 4000 volts. It drives the 8-pole generator (see center) to which it is directly connected. The generator
delivers direct current at 600 volts. It has a drum-wound armature, and its field is excited by a direct current from
a 4-pole, 125-volt exciter mounted on the extreme left end of the shaft. Motor-generator sets similar to this one are
used to furnish the current to operate electric shovels (see opposite page 120)

increase the strength of the magnetic field, and so the machine quickly builds up its current until the iron of the field magnets is brought to a state of saturation. (See opposite page 301 for a small lighting plant.)

359. The electric motor. In construction the electric motor differs in no essential respect from the dynamo. To analyze the operation as a motor of such a machine as that shown in Fig. 317, suppose a current from an outside source is first sent around the coils of the field magnets and then into the armature at b'. Here it will divide and flow through all the conductors on the left half of the ring in one direction, and through all those on the right half in the opposite direction. Hence, in accordance with the motor rule, all the conductors on the left side are urged upward by the influence of the field, and all those on the right side are urged downward. The armature will therefore begin to rotate clockwise and this rotation will continue as long as the current is sent in at b' and out at b; for as fast as coils pass either b or b' the direćtion of the current flowing through them changes, and therefore the direction of the force acting on them changes. The commutator keeps these conditions always fulfilled. The left half is therefore always urged up and the right half down. The greater the strength of the current, the greater the force acting to produce rotation.

If the armature is of the drum type (Fig. 319), the conditions are not essentially different; for, as may be seen by following out the windings, the current entering at b' will flow through all the conductors on the left half in one direction and through those on the right half in the opposite direction. (The induction motor is pictured and described opposite page 315.)

The electric motor is a device which receives electrical energy and converts it into mechanical energy. The dynamo is a device which receives mechanical energy from a steam engine, water wheel, or other source and converts it into electrical energy.

360. Street-car motors. Electric street cars are nearly all operated by direct-current series-wound motors placed under the cars and attached by gears to the axis. Fig. 322 shows a typical fourpole street-car motor. The two upper field poles are raised with the case when the

motor is opened for inspection, as in the figure. The current is generally supplied by compound-wound dynamos which maintain a constant potential of about 500 volts between the trolley or third rail and the track which is used as the return circuit. The cars are always operated in parallel, as shown in Fig. 323. In a few instances street cars are operated upon alternating, instead of upon direct-current, circuits. In such cases the motors are essentially the same as direct-current series-wound motors; for since in such a machine the current must reverse in the field magnets at Trolley wire or third rail

FIG. 322. Railway motor, upper field raised

+

Generator at power station

Track

the same time that it reverses in the armature, it will be seen that the armature is always impelled to rotate in one direction, whether it is supplied with a direct or with an alternating current. Other types of A.C. motors are not well adapted to starting with full load.

361. Back E.M.F. in motors. Group from four to six 100watt lamps in parallel and then connect the group in series with a motor of, say, horse power (the familiar 75-watt demonstration school dynamo will serve the purpose, if used as a motor.) Attach to the house circuit. As the speed of rotation of the motor increases, the lamps grow dim. If the motor is now slowed by friction at the pulley wheel, the lamps grow brighter.