Part 10

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.

VIII

APPLICATIONS OF ELECTRIC HEATING

The programme committee decided that each member of the Science Club should busy himself looking for applications of electric heating and should consult me freely about the matter. My telephone was kept busy, my laboratory was in great demand, and we were all getting a good deal more education than the school was giving us credit for.

The boys generally came to me in pairs, and each pair having worked up some illustration of heat produced by electricity reported it to the club. These were spread by the secretary in due form upon the minutes of the club and constituted "The Proceedings of the Science Club."

1. The Electric Sad Iron (Fig. 69).--Removing three screws the iron comes apart, revealing a lot of No. 24 German silver wire wound upon a sheet of mica. This is put between other sheets of mica (Fig. 70) and tucked away within the body of the iron. German silver offers about twice the resistance of iron when it is cold, but, at the temperature of the sad iron when in use, there is not much difference between the resistance of the two metals. German silver wire, however, does not rust as iron wire would, and hence it is chosen. German silver is an alloy of copper, zinc, and nickel.

We put the 112-volt current upon this wire of the iron, and according to the ammeter it passed 4 amperes. Its resistance must therefore have been 28 ohms.

(112 volts)/(28 ohms) = 4 amperes

Electricity costs us about 10 cents per kilowatt hour. That is 10 cents for 1000 watts for an hour, or 1 cent for a hundred watts for an hour, or, on a 100-volt current, 1 cent for an ampere for an hour. It, therefore, costs about 4 cents or, more accurately, 4-1/2 cents an hour to heat this iron.

Persons sometimes carry electric irons with them, when they travel, to iron pocket handkerchiefs and other small articles while stopping at a hotel. Before connecting an iron in a chandelier one must know the voltage used in the building. If the voltage in use in the building is not the same as that stamped upon the iron, it is not safe to connect it. Not knowing this, many persons have had the embarrassment of "blowing a fuse" and extinguishing their own lights, and perhaps those of others in the same building, and very likely also ruining the iron.

Suppose we take for example this iron stamped 110 V; 400 Watts. (A slight variation of 5 or 10 volts will not injure an iron.) The wire in this iron we found to offer about 28 ohms resistance when hot, and it lets pass 4 amperes. This is about all the current which it is able to carry without melting. Now suppose a 220-volt current is used in the building where it is proposed to connect the iron. This would force through the wire enough current to melt it. The wire was seen to be at a very dull-red heat when examined in a dark room. Its temperature was about nine hundred degrees. At this temperature its resistance is about three times what it is when cold. We estimated by measurements that the iron contained about twenty-five feet of the wire. The boys then took twenty-five feet of No. 24 German silver wire and stretched it between two nails driven up in the laboratory (Fig. 71, a b). The dynamo current was then sent through this. The end, c, of the wire from the dynamo was provided with a metal clip which could be slid along on the German silver wire. Sliding this to the left, and thus shortening the distance on the German silver wire through which the current must pass, increased the amount of current and heated the wire hotter. The resistance decreases as the wire is shortened.

The boys wound this wire upon a piece of asbestos board (Fig. 72), about nine inches square and one eighth of an inch thick, taking care to keep the successive turns half an inch apart. Asbestos paper was wrapped around this. The two ends of the wire were left free for connections. This they called a "hot plate."

2. Electric Hot Plate (Fig. 73).--This when opened was found to have wire coiled up inside in the same manner as the sad iron. Indeed the sad iron supported bottom side up makes a perfectly good hot plate. The particular hot plate which we examined had a three-point switch which gave three different heats for the plate. (See Fig. 74.) When the switch S is upon the first point the current goes through 112 ohms of resistance and 1 ampere passes:

(112 volts)/(112 ohms) = 1 ampere

This warms the plate slightly--enough to keep food warm which has been already cooked. This costs about one cent an hour.

When the switch is placed upon the second point the current goes through 56 ohms of resistance and 2 amperes pass.

(112 volts)/(56 ohms) = 2 amperes.

This makes the plate warmer and is adapted to certain cooking processes. It costs about two cents an hour.

When the switch is placed upon the third point the current goes through 28 ohms of resistance and 4 amperes pass.

(112 volts)/(28 ohms) = 4 amperes.

We placed upon this hot plate a basin containing 1 pint of water (equals 1 pound) and heated it from the temperature of the room (68 degrees) to boiling (212 degrees) in 7 minutes and then put an egg in and boiled it 3 minutes. Using 4 amperes for 10 minutes cost two thirds of a cent. If it takes 7 minutes to boil a pint of water it would require 1 hour to boil a gallon upon this hot plate using 4 amperes, or 448 watts. That is, it costs us about 4.5 cents a gallon to boil water by electricity. The cost is usually put at three and a half cents per gallon, but much depends upon conditions.

3. Traveller's Cooker (Fig. 75).--This consists of a hot plate with a covered basin permanently attached to it.

4. Electric Coffee Percolator (Fig. 76) consists of a hot plate with a coffee percolator to sit upon it. The coffee percolator might sit upon any other hot plate or this hot plate might serve any other purpose, but people do not seem to think of that.

5. Electric Chafing-Dish (Fig. 77) consists merely of an electric hot plate with a chafing-dish attached. The electric coffee percolators and chafing dishes require from 300 to 600 watts according to size. If used on the 110-volt current they take about 3 to 6 amperes, and if adapted to the 220-volt current they take from 1-1/2 to 3 amperes, but cost the same to operate in either case. They have connected with them flexible cords and plugs to screw into the lamp sockets.

6. Electric Broilers are merely hot plates, generally corrugated to conduct off the melted fat. One that we examined had a switch for three heats: low, requiring 360 watts--costs 3.6 cents per hour; medium, requiring 600 watts--cost 6 cents per hour; high, requiring 1280 watts--cost 12.8 cents per hour.

7. Electric Oven.--This one has double walls to retain the heat and has two large hot plates, one on the bottom and one on the top. It is large enough to hold four loaves of bread. It required 1520 watts for 40 minutes to heat it to the baking temperature and one hour to bake the bread. Hence the cost of the electricity is about 25 cents, about what the bread would cost in the market.