Fig. 68
That merely connecting the binding posts a and b ([Fig. 67]) by a small piece of wire should throw a load upon the dynamo miles away; should offer resistance to its motion, and make it require 1.18 horse-power more of energy to keep up its speed of revolution, is, indeed, uncanny. I will attempt to make it seem more real. At one end of the lecture table I have a rotary pump P ([Fig. 68]). The end of the rubber tube a, which leads to the pump is lying upon the table outside of the tank of water, T. While things are in this condition I move the crank which operates the pump with perfect ease. Now while still turning the crank I pick up the tube a and drop its free end into the water tank. I cannot now conceal the fact, even if I were disposed to do so, that I must work hard to keep the pump going. The pump itself tells you by its laboured sound that it is working hard, and the stream of water which issues from the pipe b tells how much work I am performing. The pump is discharging five and a half pints of water per second, that is 5.5 pounds, and it raises this water 10 feet. Hence I am doing 55 foot pounds of work per second, which requires one tenth of a horse-power. Here is a lad who consents to try the experiment for us. He turns the crank easily while I am holding the tube a out of the water, but when I lower it into the water he finds the resistance so great that, tug however much he may, he is unable to keep the pump going.
At the other end of the table I have a small hand dynamo, D ([Fig. 68]), M is an ammeter, V is a volt meter, S is a switch. All the wires are good-sized copper, and offer little resistance, except that stretched between the binding posts a and b. This is a piece of fine German silver wire. While the switch is open I turn the crank of the dynamo with perfect ease. A small amount of current is going through the volt meter, but this is too slight to offer any perceptible resistance to the motion of the machine.
Notice that the volt meter needle moves according to the speed of revolution. If I turn the crank once a second the needle stands at 25 volts. The electric pressure increases or decreases according to whether I rotate the armature faster or slower. Now I will attempt to keep the machine revolving at a constant rate while I close the switch S, and surely you must see that I have hard work to do so. The wire a b has now become red hot. The volt meter shows 25 volts of pressure, and the ammeter shows 3 amperes of current.
Twenty-five volts × 3 amperes = 75 watts, which require one tenth of a horse-power (746 watts = 1 horse-power). The lad now takes my place at turning the machine and finds it easy when the switch is open, but I actually overload him by merely closing the switch. Heating the wire red hot requires more energy than he is able to put forth.
I proposed to the president that my lecture close at this point, and that each one in the room have a chance to feel the load which was thrown upon the dynamo each time it was required to heat the wire. I suggested that each person should get a realizing sense of this fact, first by doing the work himself, and second by going home and reflecting upon this hint. When the switch is closed three amperes of electricity pass around the circuit. This increases the magnetism in both the field and the armature of the dynamo, and it requires one tenth of a horse-power more to keep the armature moving within the field against this magnetic pull.
I further desired to announce that during this hour I had delivered to them the second key to the Electrical Show which I had promised a few days ago. The second key is:
Heat (and light) is produced by offering resistance to the flow of the electric current. The first key is the electro-magnet. These two unlock all the mysteries of the show.
The president closed the formal exercises with the facetious remark that I had warned them before the lecture that they must work, so now each would be expected to take a turn at the cranks of the pump and dynamo.