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 220. 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 discovery of this law by Ohm, see 

 Dr. Shedd in the Popular Science Monthly for Dec., 1913). 



Briefly stated Ohm's law is: "The current in a given circuit is directly pro- 

 portional to the electromotive force, and inversely as the resistance:" 

 Nichols, p. 294. 



