Part 12

1-1/2 kilowatts × 18 hours = 27 kilowatt hours.

27 kilowatt hours at 1.5 cents = 40 cents per car day.

18. Heating Apartments by Electricity.--For heating apartments by electricity the same sort of apparatus is used as that already described for heating cars. A family of four adults, living in an eight-room apartment with at least 120 cubic feet of fresh air admitted per minute, will use on an average ten amperes of the 110-volt current. The cost will be about two dollars and fifty cents per day or seventy-five dollars per month. Although this is as much as the entire rental of a perfectly comfortable apartment, the novelty and the convenience attract tenants and the extra cost of rent does not deter them.

19. Electric Bedroom Heater.--One of the boys constructed a heater for his own room as follows: He procured a box eight inches deep by eighteen inches square on the bottom. This he lined with asbestos paper. He then stood it upon its side and arranged four incandescent light sockets as shown in Fig. 88. These were connected by a flexible cord to a plug which he could insert in place of a lamp in the chandelier. He placed this heater on the floor underneath the window and usually had 16-candle-power lamps in the sockets. He claimed that it was a jolly foot warmer and kept the room comfortable without other heat. He turned on from one to four lamps according to his need and replaced the 16-candle-power lamps by 32-candle-power lamps when the weather was extremely cold. I remarked that he must have light along with heat by this arrangement, and I should think that might be objectionable when he desired to sleep at night. He said that he always turned it off, and opened the window at night, always preferring a cold room to sleep in.

20. Cooking with Incandescent Lamps.--This piece of apparatus was devised by the boys and used in my laboratory. A sheet iron basin a, was inverted over four 16-candle-power incandescent lamps, shown in elevation by Fig. 89, and shown in plan by Fig. 90. The sides of the basin were cut so as to admit the glass globes of the lamps, but the sockets and keys were outside, so that it was convenient to turn on and off the lamps separately, thus using one half to two amperes of current, as desired. This rested upon another basin, b. Basin b was covered with asbestos for the lamps to lie on and the whole was attached to a board base, c. A flexible cord and plug allowed us to attach this to the chandelier. A pint of water was boiled upon this stove in fifteen minutes, and refreshments have been served hot from it repeatedly.

21. Electric Fireless Cooker.--There are five indictments against ordinary cooking processes.

1. They heat the house in summer.

2. They convert what would be pleasant flavours in the food into noxious odours about the house.

3. They cannot be controlled with regard to time and temperature as scientific experiments should be.

4. They confine the cook too closely and are not sufficiently automatic.

5. They are wasteful of fuel.

It would seem that electricity might enable us to cure most of these evils. To be sure the production of heat by electricity is wasteful of fuel, and it seems doubtful how the account will balance regarding the fifth item. But the remaining four items furnish a very hopeful field for research. I use the last word advisedly, and think it is just as applicable to high school boys as to university students. After experimenting awhile the boys and I concluded to give a dinner party in the laboratory and invite a few friends to test the results of our cooking.

We procured a cylinder of magnesia such as is used for covering large steam-pipes. This was inverted over our electric stove which was illustrated in Fig. 89. The magnesia was cut at the bottom, so as to give access to the key sockets of the lamps, (Fig. 91). First upon the electric stove was placed a covered dish containing a roast of lamb. Above this was another dish containing a vegetable, and upon the top of that was a pudding. A flat piece of magnesia was used as a cover to the whole. Through a hole in this was suspended a thermometer.

This "fireless cooker" was sitting in the centre of the dinner table when the guests gathered around it. We had these problems for investigation:

1. Will this cooker heat the house in summer?

All testified that they did not know that there was any heat about it until they laid their hands upon it, and then they found it only very slightly warm.

2. Is there any smell of cooking here? The process has been carried on from start to finish right on this table.

All agreed that no smell could be detected.

I then turned off the electric current which had been running until now and served the meat and vegetable, leaving the pudding inside to be kept warm by the hot walls of the cooker.

3. Regarding the control of the process: we were using 32-candle-power lamps, which gave us a variable current, from 0 to 4 amperes, and a watch and a thermometer. We had control, but as yet lacked knowledge of how it should be used. In the present case we had arbitrarily decided to begin with temperature of 400 degrees, continue it for 20 minutes, then turn off all the electric current, and let the temperature fall gradually. This had been done at our convenience in the morning before school. At a quarter before twelve we had found the temperature at 200 degrees, and turned on all the current, and now, at five minutes past twelve o'clock, all testified that the lamb was particularly good--neither too well done nor undercooked, and that its flavour was better than usual.

As for economy of fuel, we find at least that we get better results from incandescent lamps than from hot plates used in the same apparatus, and the electric equipment enables us to put the heat exactly where it is needed and nowhere else.

22. Incandescent Lamp.--We feel quite justified in putting the incandescent lamp under the heading, Applications of Electric Heating, since the electric lamps in general use convert 96 per cent. of the electric energy into heat and only 4 per cent. into light.

They were originally made by introducing a short piece of fine wire into the circuit, choosing the kind of wire, its diameter, and its length so as to make the proper relation between resistance and voltage, in order that enough current might pass to make it white hot, but not quite melt it. Platinum wire was first chosen because it would stand the highest heat without melting and without rusting.

We will pass our 112-volt current through 9 feet of the No. 24 iron wire. The wire is heated to bright red, but does not melt as it did when we used 8 feet in a former experiment. The increased length has added resistance, and, as you see by the ammeter, cut the current down from 8 to 7.5 amperes. I will now darken the room and you find that it is giving light enough to read by. But you notice that the light is growing dimmer, its colour is growing redder, and the ammeter indicates that less current is passing. I will cut off the current and let you examine the wire and you notice that a crust has formed upon it. This is due to the oxygen of the air which unites with the iron, forming iron rust. Iron rust does not conduct electricity. We have converted No. 24 iron wire into a wire of smaller diameter with a sheath of iron rust around it. We might prevent the rusting by putting the wire in a glass globe and exhausting the air from it.

I have here a piece of No. 24 platinum wire which has about the same resistance as iron wire when cold, but you notice that I may use a very much shorter length than I did of the iron wire because it will endure a very much higher heat without melting. Reducing the length would reduce the resistance, but reducing the resistance would allow more current to pass. If more current should pass it would make the wire hotter, and raising the temperature would increase the resistance, which would cut down the current, etc. By sliding the clip c (Fig. 92), along, I finally reach a point where conditions balance so that I get a very brilliant light, dangerously near the fusing point of the platinum which is three thousand degrees above the boiling point of water.

In 1879 Mr. Thomas A. Edison literally searched the whole world for something better than platinum for the filament of an incandescent lamp. He finally decided upon charred threads of a bamboo which he found in Japan. No research was ever more timely than this. Whereas there was practically no electric lighting before 1880, soon after that there began a phenomenal demand for carbon filament lamps. In 1890, 800,000 of these lamps were manufactured in the United States. In 1900 the number had risen to 25,000,000. In 1909 central stations were supplying electric current to 41,807,944 incandescent electric lights. By far the greatest number are still made with carbon filaments.

We examined an ordinary 110-volt 16-candle-power carbon filament lamp, (Fig. 93). As near as we could estimate, its filament measured about eight inches in length. We broke open the bulb of this lamp by laying it upon the table and tapping it with a board. The bulb broke with rather a loud noise and the brittle carbon filament broke into many pieces. We found one of these pieces and measured its diameter with a wire gauge, (Fig. 94). It was the same size as No. 33 wire, which we also found by the wire gauge was the size of No. 90 sewing cotton. The diameter of No. 33 wire was given upon the wire gauge as .007 inch. When lighted, the filament of this lamp had looked to be about the size of No. 18 wire, which has a diameter of .04. That is, the filament when lighted looked six times as thick as it really was. Those who use sewing cotton learn quickly to know the size of the thread by its number. So those who have much to do with wire easily learn the system of designating sizes by numbers. Here are some selected figures easy to remember. A trolley wire is about one third of an inch in diameter. It is designated as No. 0. Notice in the following table that as the numbers rise by six the diameters are divided by two. Notice also that as the diameters diminish by two the resistance increases by four.

TABLE OF RESISTANCE OF COPPER WIRES