Fig. 66
Here are two binding posts, a and b, 8 feet apart ([Fig. 66]), connected by copper wires with the dynamo circuit. The volt meter indicates 112 volts of pressure. I will close the circuit by stretching between a and b 8 feet of No. 24 iron wire. (This wire is about the thickness of a common pin.) The iron wire offers resistance to the flow of the electric current, thereby producing heat—heat enough as you see to make the wire white hot, indeed heat enough to raise it to something over two thousand degrees Fahr., for now you see it has melted.
We will put in a fresh piece of wire and connect also the ammeter in the circuit ([Fig. 67]). As I close the circuit the needle of the ammeter at first indicates 20 or 30 amperes, but in a second drops to 8 amperes, and remains there a second until the wire melts and falls apart. One hundred and twelve volts of electric pressure are able to push 8 amperes of electricity through this wire when hot.
Fig. 67
(112 volts)/(14 ohms) = 8 amperes
112 volts × 8 amperes = 896 watts
746 watts = one horse-power
Hence it required about one and one fifth horse-power to melt the wire in a second, and the heat produced was a little less than one British thermal unit, a unit much used by engineers.