[Illustration: FIG. 21.--Diagram showing simple method of carbon regulation for Arc Lamps.] As the carbon rods waste away, the length of the arc increases, and if this increase goes beyond a certain limit the arc breaks and the current ceases. If the arc is to be kept going for any length of time some arrangement for pushing the rods closer together must be provided, in order to counteract the waste. In arc lamps this pushing together, or “feeding” as it is called, is done automatically, as is also the first bringing together and separating of the rods to start or strike the arc. Fig. 21 shows a simple arrangement for this purpose. A is the positive carbon, and B the negative. C is the holder for the positive carbon, and this is connected to the rod D, which is made of soft iron. This rod is wound with two separate coils of wire as shown, coil E having a low resistance, and coil F a high one. These two coils are solenoids, and D is the core, (Chapter VII.). When the lamp is not in use, the weight of the holder keeps the positive carbon in contact with the negative carbon. When switched on, the current flows along the cable to the point H. Here it has two paths open to it, one through coil E to the positive carbon, and the other through coil F and back to the source of supply. But coil E has a much lower resistance than coil F, and so most of the current chooses the easier path through E, only a small amount of current taking the path through the other coil. Both coils are now magnetized, and E tends to draw the rod D upwards, while F tends to pull it downwards. Coil E, however, has much greater power than coil F, because a much larger amount of current is passing through it; and so it overcomes the feeble pull of F, and draws up the rod. The raising of D lifts the positive carbon away from the negative carbon, and the arc is struck. The carbons now begin to waste away, and very slowly the distance between them increases. The path of the current passing through coil E is from carbon A to carbon B by way of the arc, and as the length of the gap between A and B increases, the resistance of this path also increases. The way through coil E thus becomes less easy, and as time goes on more and more current takes the alternative path through coil F. This results in a decrease in the magnetism of E, and an increase in that of F, and at a certain point F becomes the more powerful of the two, and pulls down the rod. In this way the positive carbon is lowered and brought nearer to the negative carbon. Directly the diminishing distance between A and B reaches a certain limit, coil E once more asserts its superiority, and by overcoming the pull of F it stops the further approach of the carbons. So, by the opposing forces of the two coils, the carbons are maintained between safe limits, in spite of their wasting away. [Illustration: PLATE IX.] [Illustration: _By permission of_ _Union Electric Co. Ltd._