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 furnace and freed from small amounts of metallic sulphides, phosphides, and carbides by treatment with acid — the im- purities come from the foreign substances present in the coke and sand used.

Arc Light Furnaces. — In recent times the enormous heat developed by the arc light has been used for bringing about such chemical processes as require an extremely high temperature. This was first applied in the melting and refining of difficultly fusible metals. As there is a greater development of heat at the positive pole than at the negative the substance to be fused is placed in direct contact with the the positive pole. Many special constructions have been suggested for carrying this out. In many cases it is necessary to provide the electrodes with some form of interior cooler, in order to make them last (see p. 296).

The arc light furnaces are, however, far more important, both in industry and science, when so arranged that the heat of the arc itself is the active factor. In such furnaces use is made of the property which the arc, like all movable conductors through which a current is passing, possesses of being influenced by an electro-magnet. A conductor through which a current is passing, and which is perpen- dicular to the lines of force of a magnetic field, moves so as to cut the lines of force from right to left as seen by a person supposed to be swimming in the direction of the current, and facing in the direction opposite to that of the lines of force.

If the magnetic field is very strong, and the current producing the arc light comparatively weak, the arc may be so much affected by this attraction that it goes out. (Tesla's method of preventing a series of consecutive electric sparks from following a track is based on this phenomenon.)

The electromagnet is so arranged that the arc is attracted downwards, and thus comes into contact with the material to be heated. The longest path through which the arc is deflected is met with in Zerener's " electric blowpipe " (7^),

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