Part 10 (1/2)

[Illustration: FIG 68]

In Fig 68 the arles to the lines of force, cuts a iven 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 ar in a clockwise direction, and _y z_, therefore, isdoards Now, suppose that you rest your _left_ hand on the N

pole of the net Point your forefinger towards the S pole It will indicate the _direction of the lines of force_ Bend your other three fingers doards over the edge of the N pole They will indicate the _direction in which the conductor is ht angles to the forefinger It points in the direction in which the _induced_ current is h 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 fro C1 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 passed 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 dyna currents A well-known type of alternator is the h any one who co the brass handles connected by wires to the brushes The faster the handle of the machine is turned the er the current

[Illustration: FIG 69]

CONTINUOUS-CURRENT DYNAMOS

An alternating current is not so convenient for some purposes as a continuous current It is therefore so 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 C1,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 passed the gap on to segment C1 For half a revolution these reins to rise and _w x_ to descend, the brushes cross the gaps again and exchange seg one way through the circuit The effect of the co the brushes of the collecting rings of the alternator every tiive 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 co coil

[Illustration: FIG 70]

PRACTICAL CONTINUOUS-CURRENT DYNAMOS

The electrical output of our sile turn of wire, we used a coil ofon 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 pass through the air outside the circle described by the coil cannot be cut, and are wasted

[Illustration: FIG 71]

[Illustration: FIG 72]

The core is not a solid mass 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] Soh the core from end to end to ventilate and cool it

[Illustration: 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 fore dynaly have a number of coils wound over their drums, at equal distances round the circu is too complicated for brief treat that the coils are so connected to their respective coments and to one another that they73 will help to explain this Here we have in section a nuht of the dru, as it were, into the page), connected with conductors on the left (e) If the ”crossed” and ”dotted” conductors were respectively the ”up” and ”down” turns of a single coil ter 69), when the coil had been revolved through an angle of 90 so, so that conflicting currents would arise Yet ant to utilize the whole surface of the dru a number of coils in the manner hinted at, each coil, as it passes the zero point, top or bottoenerates a current in the desired direction and reinforces that in all the other turns of its own and of other coils on the sah the centre There is thus practically no fluctuation in the pressure of the current generated

The action of single and le and ulps; whereas the flow froed to deliver consecutively is much more constant

MULTIPOLAR DYNAMOS

Hitherto we have considered the e dynanets set inside a casing, from which their cores project towards the arnet 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 dyna brushes pick current off the commutator at equidistant points on its circumference

[Illustration: FIG 74--A Holmes continuous current dynanets]

EXCITING THE FIELD MAGNETS