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. [Illustration: Fig. 71] The boys wound this wire upon a piece of asbestos board (Fig. 72), about nine inches square and one eighth of an inch thick, taking care to keep the successive turns half an inch apart. Asbestos paper was wrapped around this. The two ends of the wire were left free for connections. This they called a "hot plate." [Illustration: Fig. 72] 2. _Electric Hot Plate_ (Fig. 73).--This when opened was found to have wire coiled up inside in the same manner as the sad iron. Indeed the sad iron supported bottom side up makes a perfectly good hot plate. The particular hot plate which we examined had a three-point switch which gave three different heats for the plate. (See Fig. 74.) When the switch _S_ is upon the first point the current goes through 112 ohms of resistance and 1 ampere passes: (112 volts)/(112 ohms) = 1 ampere [Illustration: Fig. 73] [Illustration: Fig. 74] This warms the plate slightly--enough to keep food warm which has been already cooked. This costs about one cent an hour. When the switch is placed upon the second point the current goes through 56 ohms of resistance and 2 amperes pass. (112 volts)/(56 ohms) = 2 amperes. This makes the plate warmer and is adapted to certain cooking processes. It costs about two cents an hour. When the switch is placed upon the third point the current goes through 28 ohms of resistance and 4 amperes pass. (112 volts)/(28 ohms) = 4 amperes. [Illustration: Fig. 75]