Chapter III: Preface (3)

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.

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.

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.

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

_Nos._ _Diameter_ _Resistance_

0 .32 inch 10560 feet to the ohm
6 .16 " 2640 " " " "
12 .08 " 660 " " " "
18 .04 " 165 " " " "
24 .02 " 40 " " " "
30 .01 " 10 " " " "
36 .005 " 2.5 " " " "
42 .003 " 1 " " " "

10,560 feet equal two miles.

Number 36 is the wire used upon the spools of telegraph receivers. They offer 75 ohms of resistance and therefore contain 30 feet of wire (30 × 2.5 = 75). These resistances are for ordinary school room temperatures.

Since iron has six times, and German silver twelve times the resistance of copper, divide the figures of the third column by six, and the table will answer for iron wire, or divide those figures by twelve and the table may be used for German silver wire, thus:

_Number Feet to the Ohm_
_Nos._ _Diameter_ _Copper_ _Iron_ _German Silver_

0 .32 inch 10560 1760 880
6 .16 " 2640 440 220
12 .08 " 660 110 55
18 .04 " 165 27 14
24 .02 " 40 6 32 inch
30 .01 " 10 1.5 8 "
36 .005 " 2.5 .45 2 "
42 .003 " 1 2 inch 1 "

These figures are not exact, but useful.

We procured a string of eight small lamps (Fig. 95), such as are used in lighting Christmas trees. Each was marked 14 volt, 2-candle-power. The carbon filament of each was about one inch long and apparently the same diameter as that of the 16-candle-power lamp. When the 110-volt current was sent through the group of eight connected in series they seemed to give about the same light as the single 16-candle-power lamp. It is as though the filament of the 16-candle-power lamp had been cut into eight pieces, and distributed through eight small lamps. We introduced an ammeter into the circuit and found that half an ampere of electricity passed through the single 16-candle-power lamp--and half an ampere likewise passed through the group of eight 2-candle-power lamps.

The 110-volt current can push an ampere of electricity through eight inches of carbon thread seven thousandths of an inch in diameter, and when this happens the filament gets hot enough to give out as much light as sixteen standard candles. In the place of the 16-candle-power lamp, we put a 32-candle-power 110-volt lamp. The ammeter indicated one ampere. The carbon filament was larger (No. 30, diameter = .01 inch), so as to allow more current to pass. An 8-candle-power 110-volt lamp was substituted; one quarter of an ampere passed. A 4-candle-power 110-volt lamp was used; one eighth of an ampere passed. A 100-candle-power 110-volt lamp was substituted; three amperes of current passed through it. In all these cases the lamps which passed the larger current had the larger filaments. A little practice would enable one to distinguish between these lamps without labels by examining their filaments. Among these 110-volt lamps, it is to be noted that the amount of light which they give is proportional to the amount of current which they pass. And it is convenient to remember that one ampere of electricity for one hour costs about one cent.

We introduced into the socket a "Hylo" lamp (Fig. 96). The filament, _A_, took half an ampere of electricity, gave 16-candle-power of light, and cost half a cent an hour. When the lamp was turned in its socket the current was switched off of the filament _A_, and on to the filament _a_. This took .03 of an ampere, gave one candle-power of light, and cost .03 of a cent an hour, or at the rate of about $3.00 a year, burning continuously day and night.

The uses of such a lamp are apparent in rooms which have no daylight. However, a wall switch at the entrance of such a room, making it easy to throw on and off the light entirely, seems to be a more satisfactory arrangement. One of the boys connected a wattmeter in the circuit with a hylo lamp and found that the small filament did not pass current enough to move the armature of the wattmeter. Hence that may be burned alone without affecting the consumer's bills.

We took a 16-candle-power 220-volt lamp, and lighted it by a 220-volt current. The meter showed that it allowed only one quarter of an ampere to pass. The filament was very much smaller than that in the 110-volt, 16-candle-power lamp. The pressure was twice as great as before, but the resistance was four times as great, and hence only half as much current passed. We find that it costs just as much to generate one quarter of an ampere at 220-volt pressure as it does to generate half an ampere at 110-volt pressure.

We must, of course, pay for electricity according to the cost of producing it. To produce .5 ampere at 110-volt pressure costs the same as one ampere at 55-volt pressure, or .25 amperes at 220 volts. It will be noticed that the products of the two factors in each case are the same. The product of an ampere multiplied by a volt is a watt. In each of the above three cases the amount of electrical energy is 55 watts. This will produce a definite quantity of light--about 16 candle-power when the carbon filament is used, and this quantity does not vary as either volts or amperes, but as the product of these, namely, watts.

Each of these lamps is called a 55-watt lamp, and, since they each give 16 candle-power of light, a carbon filament lamp gives one candle-power of light for three and a half watts of electricity. Electricity for lighting purposes usually costs _10 cents per kilowatt hour_, that is, 10 cents for 1000 watts for one hour, or one cent for 100 watts for one hour. Hence a 55-watt lamp costs a trifle more than half a cent for one hour, or exactly .55 cents, and a 32-candle-power lamp costs 1.1 cents per hour.

We introduced into the socket a 48-candle-power 110-volt tungsten lamp (Fig. 97), and turned on the 110-volt current. The ammeter showed 55 ampere. Hence the lamp is a 60-watt lamp, and requires one and a quarter watts per candle-power. That is, the metal tungsten is nearly three times as efficient as carbon for producing light from electricity.

With pincers we broke off the tip of a 32-candle-power carbon filament lamp, making a small hole in the large end of the bulb. The air rushed in. We then put the lamp in the socket and turned on the current. The carbon filament glowed as usual, and slowly burned up, growing smaller as it did so. The ammeter which was in circuit showed that the current, which was one ampere at the beginning, grew steadily less as the filament grew smaller, until finally when it was about one quarter of an ampere, the circuit was broken by the filament burning in two. We removed the lamp from the socket and with a dropper tube introduced a little lime water, and shook it to absorb any gas which might have been formed in there. It became milky white, as it always does when introduced where carbon has been burned. This would be a sufficient proof that the filament was made of carbon, if we did not already know it. The air is exhausted from these bulbs to prevent the carbon filament from burning up.

The carbon filament lamps were, as has been said, the invention of Mr. Thomas A. Edison in 1879. Such a statement must, however, be qualified by the assertion that this, like nearly all invention, was but the consummation of a long line of researches made by many men for many years. The early filaments were made of bamboo thread, charred, but now they are drawn like spider's web out of a sticky liquid and carbonized at a high temperature. They are attached in the lamp to short pieces of platinum wire which are sealed through the glass walls of the bulb. One wire connects with the brass collar of the bulb, and the other with the central piece of brass at the base of the bulb. We dissected a socket and found that when the lamp is placed in the socket, the collar of the lamp is screwed into the collar of the socket, and the base of the lamp comes in contact with a brass spring in the bottom of the socket (Fig. 98). The spring is connected with one copper wire bringing electricity from the dynamo. The collar is connected with the other wire from the dynamo. This connection is made and broken by turning the key of the socket. The wires are made of copper since copper is a particularly good conductor of electricity. No electricity can flow unless this circuit is complete. Socket keys and wall switches make or close gaps in this circuit. No copper wires for carrying electric-lighting current are smaller than No. 12, which has a diameter of .08 or about one twelfth of an inch. The intention is to have as little resistance to the current as possible, except in the filament of the lamp itself. There resistance is purposely introduced in order to convert electricity into light, light without heat if that were possible, but since that has not yet been found possible, heat for the sake of the accompanying light. Unhappily only 4 per cent. of the electrical energy goes into light and 96 per cent. goes into useless, or even harmful, heat. The tungsten lamps, which are now coming into use, are nearly three times as efficient in the production of light as are the carbon filament lamps. The dynamo exerts its entire pressure upon the lamp and furnishes current as follows:

A dynamo of 110-volt pressure gives:

1 ampere = 110 watts, through a 32-candle-power lamp, cost one cent an hour, or

.5 ampere = 55 watts, through a 16-candle-power lamp, cost half a cent an hour, or

.25 ampere = 27-1/2 watts, through an 8-candle-power lamp, cost a quarter of a cent an hour.

A dynamo of 220-volt pressure gives:

.5 ampere = 110 watts, through a 32-candle-power lamp, cost one cent an hour, or

.25 ampere = 55 watts, through a 16-candle-power lamp, cost half a cent an hour, or

.125 ampere = 27-1/2 watts, through an 8-candle-power lamp, cost a quarter of a cent an hour.

The carbon filament lamps, barring accidents, have a natural life varying from 600 to 1000 hours of actual incandescence. At the end of that period the filament has become so thin that it will fall apart by ordinary usage. It is never profitable, however, to use them for their whole lifetime. The lamp gradually volatilizes carbon and deposits it upon the inner walls of the bulb, producing a smoky appearance and shutting off light. As the filament grows thinner by this process, it offers greater resistance to the current, and as the amount of current grows less the proportion of light to current grows rapidly less, so that at last instead of paying for 3.5 watts of electricity per candle-power of light one must pay for perhaps seven or eight watts per candle-power. We pay fifteen cents apiece for 16-candle-power lamps, and it is economy to renew them about twice a year, if they are burned, say three hours a day, or a little over five hundred hours. It is interesting to note that when a direct current is used the evaporation from the carbon filament always takes place at the negative end alone, that is, the end from which the current is leaving the lamp. If an alternating current is used the evaporation goes on from all parts of the filament alike. This is a case of evaporation from the solid state. Carbon does not boil below 6,000 degrees, and the filament reaches about 2,450 degrees.

Tantalum, tungsten, and osmium lamps have metal filaments. These metals are better conductors than carbon but unlike carbon their resistance increases as their temperature rises, and their special virtue is that they are capable of enduring an extremely high temperature without melting. The wire used in some of these filaments is as small as .002 of an inch, or No. 44. In order to furnish sufficient resistance to prevent the 110-volt current from melting, they often have a length exceeding two feet. This is laced back and forth within the small bulb. At the temperature of bright incandescence their resistance may be increased as much as fivefold and sometimes becomes about ten ohms to the inch. Like all metals they are more brittle when cold than hot. Hence when cleaning such lamps it is advisable to turn on the current to avoid breaking the filament by jarring. Filaments which are too fragile to endure the jar of ordinary railway travel, when cold, have gone through railway wrecks safely when lighted.

It is a general rule that good conductors of electricity grow more resistant as the temperature rises while non-conductors resist less as the temperature rises. Hence the insulating material which is used to cover copper wires fails to protect if highly heated.

If a 110-volt lamp is put into a 220-volt circuit, one might expect that the lamp would burn out without doing further damage to the circuit, but this is not the case. As the filament approaches its melting point, 6000 degrees, it becomes so good a conductor that it carries current enough to melt a fifteen ampere fuse. It is, therefore, the fuse that protects the circuit and not the burning out of the lamp. The bulb containing the highly heated carbon vapour would conduct the current as an arc lamp does.

23. _Arc Lamp._--We fastened two electric light carbons to the ends of copper wires connected for the 110-volt current. A rheostat, _R_ (Fig. 99), in circuit, was set at 6.5 ohms. One lower carbon was fastened into a clamp, and the other was touched to it, and then drawn away about three-eighths of an inch. A very brilliant light was produced. Probably about 1800 candle-power. The ammeter _A_ showed 10 amperes, and the volt meter _V_ showed 45 volts. 45 volts × 10 amperes = 450 watts, 1800 candle-power, 25 watts per candle-power.

The arc light is the cheapest of all lights but is too dazzlingly bright for household purposes. It is used for outdoor lighting chiefly, and particularly for large search-lights. The temperature is over 6000 degrees, which boils the carbon and fills the gap between the two pencils with a stream of carbon vapour. This conducts the current like the filament in an incandescent lamp. The air gap between the carbon pencils would have a resistance of many thousand ohms if it were not for the presence of the carbon vapour. The hot carbon vapour reduces the resistance of this space to 4.5 ohms.

(45 volts)/(4.5 ohms) = 10 amperes.

or

(110 volts)/(6.5 + 4.5 ohms) = 10 amperes.

The carbon pencils account for part of this resistance--not more than a third of an ohm however.

It is evident that arc lamps in use must have an automatic mechanism which shall permit the carbons to touch whenever the current is not passing, but which shall draw them apart to the proper distance after the carbon vapour has been formed, or, as we say, after the arc has been established. This mechanism is nothing else than electro-magnets which are operated by the lighting circuit itself. It may require thoughtful examination to recognize these as electro-magnets, in every case, but that is what they are. Sometimes they are coils of wire, which do not have iron cores and armatures separate to be sure--but nevertheless they have both of these united in one movable rod, and they produce magnetic fields.

Suppose I pass an electric current around this coil _A_ (Fig. 100). The region about the coil becomes a magnetic field with its north pole situated at a point in space, say _N_. The influence of this field causes the iron rod to become a magnet with its south pole uppermost, and if the current is strong enough, and the field which it produces is strong enough, it will lift the iron rod up into the coil. By varying the strength of the current you see I may make this rod dance up and down in space touching nothing--a veritable ghost dance.

It may be pettifogging to say that the upper portion of this iron rod is the core of the magnetic field, and its lower portion is the armature. Yet this is right, and pettifogging may be right when it is the only way to bring out the fact.