Fig. 70
We put the 112-volt current upon this wire of the iron, and according to the ammeter it passed 4 amperes. Its resistance must therefore have been 28 ohms.
(112 volts)/(28 ohms) = 4 amperes
Electricity costs us about 10 cents per kilowatt hour. That is 10 cents for 1000 watts for an hour, or 1 cent for a hundred watts for an hour, or, on a 100-volt current, 1 cent for an ampere for an hour. It, therefore, costs about 4 cents or, more accurately, 4½ cents an hour to heat this iron.
Persons sometimes carry electric irons with them, when they travel, to iron pocket handkerchiefs and other small articles while stopping at a hotel. Before connecting an iron in a chandelier one must know the voltage used in the building. If the voltage in use in the building is not the same as that stamped upon the iron, it is not safe to connect it. Not knowing this, many persons have had the embarrassment of "blowing a fuse" and extinguishing their own lights, and perhaps those of others in the same building, and very likely also ruining the iron.
Suppose we take for example this iron stamped 110 V; 400 Watts. (A slight variation of 5 or 10 volts will not injure an iron.) The wire in this iron we found to offer about 28 ohms resistance when hot, and it lets pass 4 amperes. This is about all the current which it is able to carry without melting. Now suppose a 220-volt current is used in the building where it is proposed to connect the iron. This would force through the wire enough current to melt it. The wire was seen to be at a very dull-red heat when examined in a dark room. Its temperature was about nine hundred degrees. At this temperature its resistance is about three times what it is when cold. We estimated by measurements that the iron contained about twenty-five feet of the wire. The boys then took twenty-five feet of No. 24 German silver wire and stretched it between two nails driven up in the laboratory ([Fig. 71], a b). The dynamo current was then sent through this. The end, c, of the wire from the dynamo was provided with a metal clip which could be slid along on the German silver wire. Sliding this to the left, and thus shortening the distance on the German silver wire through which the current must pass, increased the amount of current and heated the wire hotter. The resistance decreases as the wire is shortened.
Fig. 71