Part 9

ELECTRIC HEATING

It was Washington's birthday. The schools were to have a holiday and the Science Club was to hold a special, open meeting at which I had been asked to present the subject of electricity in the household. I replied to the programme committee that that was too large a subject, but that I would talk upon electric heating. I warned them, however, that it would be a dry study, and not an entertainment. They replied that the father of his country had been born at a time of the year when the weather was unfavourable to outdoor sports, and that February usually found them acclimated to vigorous study. Neither they nor their friends objected to study if it seemed to have a motive.

I found an audience composed of old and young, men and women, girls and boys. Most of them had left school--many of them because their teachers thought they were incompetent to continue.

Not far from here is "a wheel in the middle of a wheel ... as for their rings they are so high that they are dreadful ... and the spirit of the living creature is in the wheels." Those wheels are now sending the electric current to this room for our experiments. I propose to show that we convert electricity into heat by offering resistance to its flow. Experience teaches us that resistance to motion always produces heat. At Niagara Falls thousands of tons of water descend at the rate of one hundred and sixty feet in three seconds. When the water reaches the bottom of the falls, it is moving a little faster than a mile a minute. The resistance which this mass meets after its fall retards its motion and generates heat.

Hundreds of meteors fall into our atmosphere daily, travelling a thousand times as fast as the waters of Niagara Falls. The resistance to their motion, which our atmosphere offers, heats them white hot, melts them, vaporizes them, burns them up, so that very few of them reach the solid earth in a solid condition.

An iron spile driver, measuring two cubic feet, weighs about half a ton. When it falls sixteen feet upon the end of a spile it is moving at the rate of twenty miles an hour. The energy of this moving mass depends upon both its weight and its velocity, and when its motion is arrested by the spile that energy of motion is largely converted into heat energy, from which both the spile and the spile driver get hot.

A piece of iron may be made red hot by pounding it with a trip hammer.

Count Rumford found, in 1798, while boring cannon in the arsenal at Munich, that the resistance which the iron offered to the motion of the boring tool furnished heat enough to boil water.

Seven hundred and seventy-eight foot pounds of mechanical energy when converted into heat would raise one pound of water (one pint) one degree. This is called the British thermal unit. The spile driver, weighing 1000 pounds, falling 16 feet upon a spile, produces heat enough to raise 1 pint of water 20 degrees.

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.

(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.

This piece of iron wire offered resistance to the flow of the electric current. It offered resistance to the motion of the dynamo. This offered resistance to the steam-engine which drives the dynamo. This caused the governor of the engine to open and pass more steam from the boiler. This reduced the pressure at the steam gauge. This caused the fireman to shovel more coal into the furnace. The heat of the burning coal melts the wire, but it does it only after several changes. First, it is converted into mechanical energy in the steam-engine with great loss--about nine tenths being lost. Second, it is converted into electrical energy by the dynamo, with some loss, and, third, it is conducted to the iron wire and converted back to heat with still further loss. It is evident that the most economical way to heat the wire would be to take it to the furnace. Yet all electric cooking is done by sending electric current through wires embedded in the walls of the cooking utensils, and it is the most wasteful method of using the energy stored in coal that has yet been devised.

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.