The Library of Work and Play: Electricity and Its Everyday Uses — Reading Notes

inally 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.

(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.

Our great study now is to produce light without heat, or at least to come as near to it as the firefly does. The firefly gives 98 per cent. light and two per cent. heat. The arc lamp gives 12 per cent. light and 88 per cent. heat. The carbon filament gives 4 per cent. light and 96 per cent. heat. When we have made considerable progress in that direction we shall take electric lamps out of the chapter on electric heating and form a new chapter on electric lighting.