Fig. 72
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."
Fig. 73
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