current is used, of low voltage but very high amperage. Steel castings are sometimes found to have small defects, such as cracks or blow-holes. These are not discarded as useless, but are made quite sound by welding additional metal into the defective places by means of the electric arc. The arc is formed between the casting and a carbon rod, and the tremendous heat reduces the surface of the metal to a molten condition. Small pieces or rods of metal are then welded in where required. CHAPTER XIV ELECTRIC BELLS AND ALARMS The most familiar of all electrically worked appliances is probably the electric bell, which in some form or other is in use practically all over the world. Electric bells are operated by means of a current of electricity sent through the coils of an electro-magnet, and one of the very simplest forms is that known as the single-stroke bell. In this bell an armature or piece of soft iron is placed across, but at a little distance from, the poles of an electro-magnet, and to this piece of iron is fixed a lever terminating in a sort of knob which lies close to a bell or gong. When a current is sent round the electro-magnet the armature is attracted, so that the lever moves forward and strikes a sharp blow upon the gong. Before the gong can be sounded a second time the current must be interrupted in order to make the magnet release the armature, so that the lever may fall back to its original position. Thus the bell gives only one ring each time the circuit is closed. Bells of this kind may be used for signalling in exactly the same way as the Morse sounder, and sometimes they are made with two gongs of different tones, which are arranged so as to be sounded alternately. [Illustration: FIG. 24.--Mechanism of Electric Bell.] [Illustration: FIG. 25.--Diagram showing principle of Bell-push.]