[Ill.u.s.tration: FIG. 68.]

In Fig. 68 the armature, moving at right angles to the lines of force, cuts a maximum number in a given time, and the current induced in the coil is therefore now most intense. Here we must stop a moment to consider how to decide in which direction the current flows. The armature is revolving in a clockwise direction, and _y z_, therefore, is moving downwards. Now, suppose that you rest your _left_ hand on the N.

pole of the magnet so that the arm lies in a line with the magnet. Point your forefinger towards the S. pole. It will indicate the _direction of the lines of force_. Bend your other three fingers downwards over the edge of the N. pole. They will indicate the _direction in which the conductor is moving_ across the magnetic field. Stick out the thumb at right angles to the forefinger. It points in the direction in which the _induced_ current is moving through the nearer half of the coil.

Therefore lines of force, conductor, and induced current travel in planes which, like the top and two adjacent sides of a box, are at right angles to one another.

While current travels from _z_ to _y_--that is, _from_ the ring C^1 to _y_--it also travels from _x_ to _w_, because _w x_ rises while _y z_ descends. So that a current circulates through the coil and the exterior part of the circuit, including the lamp. After _z y_ has pa.s.sed the lowest possible point of the circle it begins to ascend, _w x_ to descend. The direction of the current is therefore reversed; and as the change is repeated every half-revolution this form of dynamo is called an _alternator_ or creator of alternating currents. A well-known type of alternator is the magneto machine which sends shocks through any one who completes the external circuit by holding the bra.s.s handles connected by wires to the brushes. The faster the handle of the machine is turned the more frequent is the alternation, and the stronger the current.

[Ill.u.s.tration: FIG. 69.]

CONTINUOUS-CURRENT DYNAMOS.

An alternating current is not so convenient for some purposes as a continuous current. It is therefore sometimes desirable (even necessary) to convert the alternating into a uni-directional or continuous current.

How this is done is shown in Figs. 69 and 70. In place of the two collecting rings C C^1, we now have a single ring split longitudinally into two portions, one of which is connected to each end of the coil _w x y z_. In Fig. 69 brush B has just pa.s.sed the gap on to segment C, brush B^1 on to segment C^1. For half a revolution these remain respectively in contact; then, just as _y z_ begins to rise and _w x_ to descend, the brushes cross the gaps again and exchange segments, so that the current is perpetually flowing one way through the circuit. The effect of the commutator[17] is, in fact, equivalent to transposing the brushes of the collecting rings of the alternator every time the coil reaches a zero position.

Figs. 71 and 72 give end views in section of the coil and the commutator, with the coil in the position of minimum and maximum efficiency. The arrow denotes the direction of movement; the double dotted lines the commutator end of the revolving coil.

[Ill.u.s.tration: FIG. 70.]

PRACTICAL CONTINUOUS-CURRENT DYNAMOS.

The electrical output of our simple dynamo would be increased if, instead of a single turn of wire, we used a coil of many turns. A further improvement would result from mounting on the shaft, inside the coil, a core or drum of iron, to entice the lines of force within reach of the revolving coil. It is evident that any lines which pa.s.s through the air outside the circle described by the coil cannot be cut, and are wasted.

[Ill.u.s.tration: FIG. 71.]

[Ill.u.s.tration: FIG. 72.]

The core is not a solid ma.s.s of iron, but built up of a number of very thin iron discs threaded on the shaft and insulated from one another to prevent electric eddies, which would interfere with the induced current in the conductor.[18] Sometimes there are openings through the core from end to end to ventilate and cool it.

[Ill.u.s.tration: FIG. 73.]

We have already noticed that in the case of a single coil the current rises and falls in a series of pulsations. Such a form of armature would be unsuitable for large dynamos, which accordingly have a number of coils wound over their drums, at equal distances round the circ.u.mference, and a commutator divided into an equal number of segments. The subject of drum winding is too complicated for brief treatment, and we must therefore be content with noticing that the coils are so connected to their respective commutator segments and to one another that they mutually a.s.sist one another. A glance at Fig. 73 will help to explain this. Here we have in section a number of conductors on the right of the drum (marked with a cross to show that current is moving, as it were, into the page), connected with conductors on the left (marked with a dot to signify current coming out of the page). If the "crossed" and "dotted" conductors were respectively the "up" and "down" turns of a single coil terminating in a simple split commutator (Fig. 69), when the coil had been revolved through an angle of 90 some of the up turns would be ascending and some descending, so that conflicting currents would arise. Yet we want to utilize the whole surface of the drum; and by winding a number of coils in the manner hinted at, each coil, as it pa.s.ses the zero point, top or bottom, at once generates a current in the desired direction and reinforces that in all the other turns of its own and of other coils on the same side of a line drawn vertically through the centre. There is thus practically no fluctuation in the pressure of the current generated.

The action of single and multiple coil windings may be compared to that of single and multiple pumps. Water is ejected by a single pump in gulps; whereas the flow from a pipe fed by several pumps arranged to deliver consecutively is much more constant.

MULTIPOLAR DYNAMOS.

Hitherto we have considered the magnetic field produced by one bi-polar magnet only. Large dynamos have four, six, eight, or more field magnets set inside a casing, from which their cores project towards the armature so as almost to touch it (Fig. 74). The magnet coils are wound to give N. and S. poles alternately at their armature ends round the field; and the lines of force from each N. pole stream each way to the two adjacent S. poles across the path of the armature coils. In dynamos of this kind several pairs of collecting brushes pick current off the commutator at equidistant points on its circ.u.mference.

[Ill.u.s.tration: FIG. 74.--A Holmes continuous current dynamo: A, armature; C, commutator; M, field magnets.]

EXCITING THE FIELD MAGNETS.

Until current pa.s.ses through the field magnet coils, no magnetic field can be created. How are the coils supplied with current? A dynamo, starting for the first time, is excited by a current from an outside source; but when it has once begun to generate current it feeds its magnets itself, and ever afterwards will be self-exciting,[19] owing to the residual magnetism left in the magnet cores.

[Ill.u.s.tration: FIG. 75.--Partly finished commutator.]

Look carefully at Figs. 77 and 78. In the first of these you will observe that part of the wire forming the external circuit is wound round the arms of the field magnet. This is called a _series_ winding.

In this case _all_ the current generated helps to excite the dynamo. At the start the residual magnetism of the magnet cores gives a weak field.

The armature coils cut this and pa.s.s a current through the circuit. The magnets are further excited, and the field becomes stronger; and so on till the dynamo is developing full power. Series winding is used where the current in the external circuit is required to be very constant.

[Ill.u.s.tration: FIG. 76.--The brushes of a Holmes dynamo.]

Fig. 78 shows another method of winding--the _shunt_. Most of the current generated pa.s.ses through the external circuit 2, 2; but a part is switched through a separate winding for the magnets, denoted by the fine wire 1, 1. Here the strength of the magnetism does not vary directly with the current, as only a small part of the current serves the magnets. The shunt winding is therefore used where the voltage (or pressure) must be constant.

[Ill.u.s.tration: FIG. 77.--Sketch showing a "series" winding.]

[Ill.u.s.tration: FIG. 78.--"Shunt" winding.]

A third method is a combination of the two already named. A winding of fine wire pa.s.ses from brush to brush round the magnets; and there is also a series winding as in Fig. 77. This compound method is adapted more especially for electric traction.

ALTERNATING DYNAMOS.

These have their field magnets excited by a separate continuous current dynamo of small size. The field magnets usually revolve inside a fixed armature (the reverse of the arrangement in a direct-current generator); or there may be a fixed central armature and field magnets revolving outside it. This latter arrangement is found in the great power stations at Niagara Falls, where the enormous field-rings are mounted on the top ends of vertical shafts, driven by water-turbines at the bottom of pits 178 feet deep, down which water is led to the turbines through great pipes, or penstocks. The weight of each shaft and the field-ring attached totals about thirty-five tons. This ma.s.s revolves 250 times a minute, and 5,000 horse power is constantly developed by the dynamo.

Similar dynamos of 10,000 horse power each have been installed on the Canadian side of the Falls.

[Ill.u.s.tration: FIG. 79.]

TRANSMISSION OF POWER.

Alternating current is used where power has to be transmitted for long distances, because such a current can be intensified, or stepped up, by a transformer somewhat similar in principle to a Ruhmkorff coil _minus_ a contact-breaker (see p. 122). A typical example of transformation is seen in Fig. 79. Alternating current of 5,000 volts pressure is produced in the generating station and sent through conductors to a distant station, where a transformer, B, reduces the pressure to 500 volts to drive an alternating motor, C, which in turn operates a direct current dynamo, D. This dynamo has its + terminal connected with the insulated or "live" rail of an electric railway, and its - terminal with the wheel rails, which are metallically united at the joints to act as a "return." On its way from the live rail to the return the current pa.s.ses through the motors. In the case of trams the conductor is either a cable carried overhead on standards, from which it pa.s.ses to the motor through a trolley arm, or a rail laid underground in a conduit between the rails. In the top of the conduit is a slit through which an arm carrying a contact shoe on the end projects from the car. The shoe rubs continuously on the live rail as the car moves.

To return for a moment to the question of transformation of current.

"Why," it may be asked, "should we not send low-pressure _direct_ current to a distant station straight from the dynamo, instead of altering its nature and pressure? Or, at any rate, why not use high-pressure direct current, and transform _that_?" The answer is, that to transmit a large amount of electrical energy at low pressure (or voltage) would necessitate large volume (or _amperage_) and a big and expensive copper conductor to carry it. High-pressure direct current is not easily generated, since the sparking at the collecting brushes as they pa.s.s over the commutator segments gives trouble. So engineers prefer high-pressure alternating current, which is easily produced, and can be sent through a small and inexpensive conductor with little loss.

Also its voltage can be transformed by apparatus having no revolving parts.

THE ELECTRIC MOTOR.

Anybody who understands the dynamo will also be able to understand the electric motor, which is merely a reversed dynamo.

Imagine in Fig. 70 a dynamo taking the place of the lamp and pa.s.sing current through the brushes and commutator into the coil _w x y z_. Now, any coil through which current pa.s.ses becomes a magnet with N. and S.

poles at either end. (In Fig. 70 we will a.s.sume that the N. pole is below and the S. pole above the coil.) The coil poles therefore try to seek the contrary poles of the permanent magnet, and the coil revolves until its S. pole faces the N. of the magnet, and _vice versa_. The lines of force of the coil and the magnet are now parallel. But the momentum of revolution carries the coil on, and suddenly the commutator reverses its polarity, and a further half-revolution takes place. Then comes a further reversal, and so on _ad infinitum_. The rotation of the motor is therefore merely a question of repulsion and attraction of like and unlike poles. An ordinary compa.s.s needle may be converted into a tiny motor by presenting the N. and S. poles of a magnet to its S. and N. poles alternately every half-revolution.

In construction and winding a motor is practically the same as a dynamo.

In fact, either machine can perform either function, though perhaps not equally well adapted for both. Motors may be run with direct or alternating current, according to their construction.

On electric cars the motor is generally suspended from the wheel truck, and a small pinion on the armature shaft gears with a large pinion on a wheel axle. One great advantage of electric traction is that every vehicle of a train can carry its own motor, so that the whole weight of the train may be used to get a grip on the rails when starting. Where a single steam locomotive is used, the adhesion of its driving-wheels only is available for overcoming the inertia of the load; and the whole strain of starting is thrown on to the foremost couplings. Other advantages may be summed up as follows:--(1) Ease of starting and rapid acceleration; (2) absence of waste of energy (in the shape of burning fuel) when the vehicles are at rest; (3) absence of smoke and smell.

ELECTRIC LIGHTING.

Dynamos are used to generate current for two main purposes--(1) To supply power to motors of all kinds; (2) to light our houses, factories, and streets. In private houses and theatres incandescent lamps are generally used; in the open air, in shops, and in larger buildings, such as railway stations, the arc lamp is more often found.