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CH. XIII] RHEOSTATS AND OTHER BALLAST 521
quickly replaced. To replace a fuse, open the nearest switch which will turn off the current from the line. Take out both fuses, and examine them ; only one is likely to have melted. It is usually easy to tell which. Discard that one, then insert two good fuses of the proper capacity, close the switch, and the current will be available again.
If the lights on a particular line go out from the blowing of a fuse, and one is not sure which branch it is in the fuse box, the one is easily found by using the testing lamp (fig. 21) beyond the fuses. The lamp will light on all the lines with perfect fuses when put across the blades of the special line switch, or when put in contact with any naked metal part across the line. The line with a burned out fuse will not light the testing lamp, when it is applied beyond the fuse.
RESISTORS OR RHEOSTATS: INSTALLATION AND USE
§ 723. Resistor or rheostat. — A rheostat is a conductor having considerable resistance ; it is placed in an electric circuit to regulate the amount of current. In passing through the rheostat much heat is developed by the energy consumed in overcoming the resistance. This energy consumption is a dead loss.
The conductor used is ordinarily in the form of wire or strips of metal such as German silver, iron or nickel.
§ 724. Amount of resistance needed. — Electricians have worked out with much accuracy the resistance of different metals and by consulting the tables furnished in books on electrical engineering one can find how great a length of a given size iron or German silver wire is necessary to afford the proper resistance for any given constant voltage, as no or' 2 20. See §
§ 724a. Ohm's Law and its application to projection apparatus. — While the units, volt, ampere and ohm (§ 654-657) might be worth defining, still it would lead to no very practical results unless there was a definite relation between the electric quantities for which these units stand.
It has been found by experiment that there is a very definite relationship, known as Ohm's Law. (For a history of the djio^very of this law by Ohm, see Dr. Shedd in the Popular Science Monthly forTDec., 1913).
Briefly stated Ohm's law is: "The current in a given circuit is directly proportional to the electromotive force, and inversely as the resistance:" Nichols, p. 294.