Part 11

8. Electric Incubator.--This is simply a well-ventilated oven warmed by an electric hot plate and automatically controlled so that it keeps a constant temperature of 103 degrees. Under these conditions chickens hatch from hens' eggs in three weeks. An incubator for 5 dozen eggs was found to take 25 cents' worth of electricity for the whole process of incubation.

9. Electric Toaster.--The wire coiled up in sad irons and hot plates becomes hot enough to scorch cloth and paper, and even set fire to them if they come in direct contact. We proved this by opening the iron and touching paper to the wire while it was carrying the current. We also lighted a cigar by touching it to the wire. Electric toasters have the hot German silver wire simply covered by a screen.

10. Electric Cigar Lighters (Fig. 78).--The one we examined hung by a flexible cord from the chandelier. It had a small disk on the side which contained a lot of fine wire covered by perforated mica. The wire became red hot when the push button in the handle was pressed. It took half an ampere of 110-volt current, and operated only while the button was pushed. As near as we could calculate it cost .0003 of a cent to light a cigar.

11. Electric Curling Iron (Fig. 79).--One who has flat hair needs no curling iron, but those who have round hair may curl it temporarily, if they will unscrew an electric light bulb and screw into its socket the plug of an electric curling iron. The flexible cord contains two wires insulated from each other. One of these wires is attached to the outer shell of the plug, the other wire is attached to the central button of the plug. These make connections with the two separate dynamo wires in the socket. The current comes down one of the wires in the flexible cord, passes through a coil of fine German silver wire inside of the curling iron, and returns by the other wire in the flexible cord. The small wire in the curling iron offers 220 ohms of resistance when hot and passes half an ampere of the 110-volt current.

(110 volts)/(220 ohms) = .5 ampere.

12. Electric Soldering Irons (Fig. 80).--Or coppers, as they should be called, are ideal implements for soldering. They remain continually at the proper temperature and are free from corrosion. They require from 55 to 220 watts. On the 110-volt current they take from one half to two amperes.

13. Electric Heating Pad (Fig. 81).--This consists of resistance wire inside of a pad of soft material. It maintains a temperature of 180 degrees, and is an excellent substitute for a hot water bag. It contains about two hundred and twenty ohms of resistance and requires the same current as a 16-candle-power lamp.

14. Electric Fuses (Fig. 82).--Fuses are made of short pieces of wire or thin sheet metal. The metal is an alloy of lead and tin which melts at a low temperature. They derive their name from the fact that they readily fuse or melt. A building is wired in various separate circuits. The size of the copper wires used in each circuit is determined by the amount of current which the circuit is expected to carry. Each circuit is protected by one or more fuses. These melt and cut off the current whenever too much passes for the copper conductor to carry without getting hot. The fuse wire melts at about six hundred degrees, while the copper will not melt until it reaches nearly two thousand degrees. This temperature is sufficient to set fire to wood, paper, and cloth. When any fuse melts, the current is cut off from all chandeliers, etc., in the particular circuit controlled by the fuse. This produces consternation among people who do not understand the function of a fuse. They become panic-stricken and begin to trample their neighbours to death in the theatre or on the electric train when they hear that a fuse is "blown" (which is the electrician's way of saying that it has melted). Everyone should know that a fuse is a safety device. It is always enclosed in a box lined with sheet iron or asbestos, so that it is impossible for the flash, which occurs when the circuit is broken, to set fire to anything.

15. Electric Gas Lighter (Fig. 83).--These usually have two or three small, dry battery cells in the handle. By pushing a button in the handle connection is made between this battery and a short piece of resistance wire in the tip. This wire gets red hot and lights the gas. It is a surprise to many that we can light illuminating gas without bringing a flame to it, and it is equally surprising that some flames, or at least sparks, may not be able to light the gas. The fact is that it is wholly a matter of temperature and kind of gas. Iron heated to dull red will not light the illuminating gas now being furnished in New York City, while iron at a bright red heat will do so. Iron may be hot enough to light illuminating gas but too cool to light gasolene vapour, which requires a dazzling white heat. Iron which is just under the temperature at which it gives any light may set fire to wood and paper. After it has cooled a good deal below that, it will set fire to sulphur, and when it has cooled so that one may hold it in the hand, it is still hot enough to set fire to phosphorus. The glowing end of a lighted cigar, the spark made by striking flint, or the spark from a spark coil with a feeble battery, all fail to set fire to gasolene vapour, simply because they are not hot enough.

Fresh battery cells must occasionally be put in the handle of the electric gas lighter.

Four facts regarding the resistance of wires it is well to remember:

1. The longer the wire the more resistance it offers to the electric current.

2. The smaller the diameter of the wire the more resistance it offers.

3. Some materials offer more resistance than others, for example, iron about six times as much as copper and German silver about twelve times as much as copper.

4. The common metals offer more resistance when hot than when cold, about double the resistance when heated to five hundred degrees. It is the reverse with carbon, which offers more resistance when cold than when hot. The carbon filament lamp offers about double the resistance when cold as when lighted to full brilliancy.

16. Electric Flasher (Fig. 84).--For automatically flashing electric lights. The one which we examined was constructed according to the plan shown in Fig. 85. The lighting circuit is brought to the binding posts b and c. A small insulated wire of high resistance connects b and c, being wound around the metal bar a b. The resistance of this wire, when added to that of lamps, permits not more than one fifth of an ampere to pass, and this warms the wire slightly. The bar a b is composed of two strips of metal, brass above and iron below. Heat expands brass more than iron. The result is that when the current is turned on, the bar begins to curve downward until presently it touches the metal base of c. Then the full current required to light the lamps which are in circuit passes. While the circuit is closed through the large metal strips not enough passes through the fine wire to warm it. On cooling, a b curves upward and breaks the connection with c, and now the current begins again to warm up the small wire.

The flasher that we examined was adapted to operate: one 32-candle-power lamp; or two 16-candle-power lamps; or four 8-candle-power lamps, on a one ampere circuit of 110-volt pressure.

Let us see what would happen if it were connected either with a current of higher voltage or a circuit of more lamps. Suppose we have a 32-candle-power carbon filament lamp in circuit. This requires one ampere to light it. Its resistance when hot is 110 ohms.

(110 volts)/(110 ohms) = 1 ampere.

When cold its resistance is about double or 220 ohms. The German silver wire of the electric flasher offers 330 ohms of resistance, and together they make 550 ohms. Thus the current is cut down to .2 ampere.

(110 volts)/(330 + 220 ohms) = .2 ampere

Suppose now we should undertake to use the same flasher and the same lamp on a 220-volt current. This might push more current through than the small wire could carry. It might melt, or its insulation might burn off before a made contact with b; if not the lamp would certainly burn out after the contact. If we undertook to operate with this flasher several 32-candle-power lamps instead of one upon the 110-volt circuit, the result would be the same, for in that case the resistance would be reduced and, therefore, a greater current would pass than the wire could carry without undue heating.

The boys were at first troubled to see how increasing the number of lamps in a circuit would decrease the resistance in that circuit. Fig. 86 was drawn to explain the matter. The lamps l, l, l, etc., are connected in parallel. Each lamp makes an independent connection from one feed wire to the other. The flasher a acts as a switch to close the circuit for the whole.

Now if we think of these wires as pipes to conduct water we would say that water flows from D to E through ten pipes more readily than through one. It would meet with only one tenth as much resistance. The result would be the same, if we should substitute for the ten pipes one pipe ten times as large in cross section. So it is with wires which are conducting electricity. Introduce two in parallel, and you allow twice as much current to pass by reducing the resistance to one half. Ten parallel conductors reduce the resistance to one tenth and allow ten times as much current to pass.

It is to be noticed that this flasher is an automatic switch which is opened or closed according to temperature. Remove the fine wire from a and we have precisely the device which regulated the temperature in our electric incubator. Suppose the "thermostat" (as it is called in that case) is placed within the egg chamber which is to be kept at 103 degrees. A screw in the metal strip c underneath the end of a may be set so that it will normally touch a. Suppose now the brass strip is underneath the strip of iron in a. As the hot plate warms up the egg chamber, the brass will expand more than the iron, and the bar will curve upward and break the connection with c. As soon as the current stops the temperature of the chamber begins to fall, and the bar curves downward again until connection is made. This device is capable of adjustment so as to keep the temperature constantly at 103 degrees or any other desired degree. The device is in use for scores of different purposes, including the regulation of temperature in school rooms.

17. Electric Car Heaters.--Ten or fifteen years ago there were no heated street cars in New York City. Now they are all heated by electricity and their maximum and minimum temperatures are regulated by law. The resistance wire may be seen in coils underneath the car seats. Electric street cars usually operate on a 500 or 600-volt current. The amount of current used for heating varies from 2 to 12 amperes. Perhaps 3 amperes may be taken as an average.

500 V × 3 a = 1500 w = 1-1/2 kilowatts.

It costs the large electric railway companies about 1.5 cents per kilowatt hour to generate their supply of current. Eighteen hours is considered a car day.