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CH. XIII] RHEOSTATS AND OTHER BALLAST 525
In wiring the rheostat, it is to be placed in one wire, (in series) as all the current must pass through it (fig. 1 88, 281). It makes no difference whether it is placed in the wire going to the upper carbon or coming from the lower carbon.
§ 729. Calibration of a rheostat. — The maker of a rheostat should mark plainly upon it its capacity if it is of the fixed form. If it is adjustable, then the range of the rheostat should be given.
Furthermore, the lower range should be plainly marked at the lowest step and the highest range at the highest step. The user of a rheostat like that in fig. 145 could not tell easily which way to turn the knob to increase or diminish the current unless the maker indicates the amperage at the two ends of the steps. In case there is no indication a person can determine it for himself if he has an ammeter.
Insert the ammeter in one wire of the line (fig. 273). Turn the knob of the rheostat to the middle step, insert proper carbons in the arc lamp, and turn on the current. When the lamp is burning properly note the reading on the ammeter. Turn the knob toward one side and the ammeter will indicate whether there is more or less current. One can in this way find the amount of current delivered for the different positions. It is well to mark on the rheostat face with white paint the amperages corresponding to these positions. It is also a help to have an arrow pointing from the lowest to the highest amperage (fig. 182, 281).
§ 730. Home-made rheostats. — While it is altogether false economy to use anything but the best in the form of a rheostat it is worth while knowing how one could be made in case of urgent need.
§ 73 1 . Barrel or bucket type of salt water rheostat. — A wooden bucket or barrel is used. In the bottom is placed a large plate of iron, and one end of the supply wire is firmly fixed to this. The other end of the wire is fixed to another mass of iron. The barrel or bucket is then filled nearly full of water, and enough common salt added to make about a >£% solution. The water should be well stirred to evenly distribute the salt. The upper iron and