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HANDBOOK OF PROJECTION FOR
such as are used for mounting electric circuit wires. The ordinary porcelain knob insulators will do. These extra coils may be mounted on a brick wall or on a suitable iron frame, but they must not be placed near anything inflammable ; they must also be protected by an outer casing the same as the regular rheostat coils. Wire screening having about Y^ inch mesh is suitable for protection. These coils may either be mounted near the rheostat, or at some distance from it, connection being made between them and the rheostat by means of a suitable size copper wire.
IRON WIRE RHEOSTATS.— It is possible to construct a rheostat from ordinary iron wire, but such wire has a very high temperature coefficient, which means that its resistance
Figure 133.
increases rapidly with the increase of temperature. The result of this is that if you build an iron wire rheostat capable of delivering the amperage you want after it becomes hot, it will give altogether too much when you first strike the arc.
OBJECTION TO BIG GRID RHEOSTATS.— As has been said, the temperature coefficient of cast iron is high. This means that a cast iron conductor of given area will deliver very much more current when it is cold than when it is hot. This forms one of the objectionable features of the cast grid rheostat, but the objection is not serious when rheostats of small size are considered. When we come to consider the cast grid rheostat of large size, however, it