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. [Illustration: 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. 1 pound raised 1 foot = 1 foot pound 550 foot pounds per second = 1 horse-power 778 foot pounds (1.4 H.-P.) = 1 B. T. U. (British thermal unit) = heat required to raise 1 pound of water 1° Fahrenheit 1 volt × 1 ampere = 1 watt 746 watts = 1 horse-power In order to hold back 112 volts of electric pressure so that not more than eight amperes of electricity should pass, the iron wire must have offered about 14 ohms of resistance. The behaviour of the ammeter needle showed that the wire offered very much less resistance when cold than when hot. Indeed eight feet of No. 24 iron wire offers about one and one third ohms resistance when cold, hence heat had increased its resistance to the passage of the electric current tenfold.