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 = 27½ 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 = 27½ 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.

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.

One might expect that a rod made of carbon would quickly burn up, particularly when raised to the exceeding high temperature of the electric arc. While it is true that carbon in the form of charcoal burns so readily that it is used instead of kindlings for lighting a fire, carbon in the form of graphite in our so-called "lead" pencils and carbon as it is prepared for electric light pencils burns only very slowly even at exceedingly high temperatures. The carbon rods used in arc lamps endure a temperature of over 6000 degrees, without losing more than one inch an hour, and half of that is simply volatilized—not burned.

One of the most interesting improvements ever made in the arc light is that of enclosing the arc in an inner glass globe. This globe is closed airtight below with a small opening above. When the arc is formed the oxygen of the air in the inner globe is soon consumed and then combustion is no longer possible. We illustrated this by an experiment. An ordinary cork was chosen to fit the large end of an argand lamp chimney and through a hole in this was passed one of the carbon rods (Fig. 101). A metal clamp made connections between this carbon and the negative wire from the dynamo. The other carbon, attached by a clamp to the positive wire, was thrust down into the upper end of the chimney until it touched the negative carbon, and then drawn upward a short distance, drawing an arc, as we say. This soon makes an atmosphere within the chimney where combustion cannot go on for want of oxygen. The arc, however, continues to glow as in the open air, and the carbons may be drawn further apart than in the open air without breaking the arc, hence more of the external resistance may be cut out and a higher voltage put upon the lamp.

Carbons which burn out in a single night if used in open arc lamps last two weeks in enclosed arc lamps.

The lower carbon, when removed from the lamp chimney of the last experiment, served as a lead pencil to write on paper. The positive carbon would not make a mark on paper. In all arc lamps carbon is distilled from the positive pencil, condensing upon the negative pencil as graphite, which is the material used in making "lead" pencils. They are called "lead" pencils because they were originally made of lead, but now they are made of graphite which is mined from the earth.

As soon as the arc is broken it becomes evident that the positive carbon has been heated much the hotter of the two, a fact that could not be detected while it was lighted because of the dazzling brightness of the arc. The negative carbon turns black almost immediately, while the positive carbon remains at a bright red heat for some time.

This fact needs to be borne in mind when adjusting arc light carbons in search-lights, stereopticons, and all like apparatus in which the light must be placed at the focus of a lens. That is, it is necessary to know from what point the light really comes and it is necessary to have some adjusting device to keep this point continually at the focus of the lens.

24. Search-Light.—(Fig. 102). This is simply an arc lamp with reflectors behind it and lenses in front of it. The whole apparatus is pivoted so as to be easily made to shine in any direction. The function of the lenses and the reflectors is to collect stray rays of light and send them all out in the same direction. This is shown in Fig. 103 where for simplicity the lens is represented as a single piece. L represents a point of light which will naturally send its rays out in all directions as the radii of a sphere; m, m, m represents a bright reflecting surface which is given that peculiar curve called a parabola. It has the unique faculty of reflecting in a parallel direction all the rays which may fall upon it from L, so long as L is kept at that particular point called the focus, a b is a lens of glass which has that peculiar curve that enables it to bend all rays which fall upon it from L, so that they may pass out parallel.

25. Stereopticon.—This also has the necessary devices to gather the rays of the arc lamp and send them forth parallel, and in addition it has a series of lenses which produce upon a distant screen an enlarged picture of any transparent object held in these parallel rays.

26. Burglar's Flash-Light.—There are many forms of this. The one we examined is represented in Fig. 104. We unscrewed a metal ring at the left-hand end and found, first a glass lens and behind that a miniature electric light, requiring three volts and half an ampere. We knew, therefore, that it must be supplied with two cells, since one cell may give not more than 1.5 volts. We also knew that it would only be used to flash a light, since if dry cells are required to furnish half an ampere continuously they soon run down. Behind the lamp there was a bright metal reflector—the lens and reflector are fairly well represented in Fig. 103. The filament of the lamp is connected with two small battery cells in the handle. These may be removed and replaced by new ones by unscrewing a cap at the right-hand end. The circuit is closed by a metal spring on the side of the tube, which acts as a push button. It is situated where it may be conveniently pressed by the thumb. The small batteries necessarily have a short life and must be replaced quite frequently. Being a special thing they cost nearly twice what the regular dry cell does.

27. Mercury Vapour Lamp.—This is an interesting variety of arc light in which the vapour of mercury takes the place of the vapour of carbon. G, in Fig. 105, represents a glass tube from which the air has been exhausted. The wires of the lighting circuit are fused into the ends of the tube. At one end, and in contact with one of these wires, is a small pool of mercury. By pulling the cord c the tube is tilted on the pivot p, so that a stream of mercury flows along the whole length of the tube and closes the electric circuit. When the tube falls back into its normal position, as represented in the figure, the electric arc persists upon the mercury vapour. Incandescent mercury vapour gives light strong in green, blue, and violet, but deficient in red and yellow. It, therefore, gives nothing its natural appearance but casts a ghastly hue over everything.

This lamp was invented in 1902, by Peter Cooper-Hewitt, grandson of the founder of Cooper Union in New York City.

It gives a very suitable light for making photographic prints, and is much used for that. This lamp operates upon the 110-volt circuit. It is the longest step yet taken toward getting light without heat, but perhaps shows what we must expect when we reach that goal, namely, unsatisfactory colour values in the light. Probably such is the case with the firefly.

28. The Moore Light.—In 1896 Prof. D. McFarland Moore brought out his vacuum tube light (Fig. 106). We visited an ordinary dry goods store which had been equipped with this. Glass tubing is put together very much as one would put up a stove pipe or a job of plumbing. The joints are fused and made air-tight by playing a flame upon them after the pipe is up in place. This pipe is led around into all nooks and corners where there would be dark places. The air is pumped out of this tube and a trifling amount of some vapour is introduced, the kind varying according to the tint of colour which is desired.

29. The Nernst Lamp.—In 1897 the Nernst lamp appeared in Germany. It is a good illustration of an insulating substance becoming a conductor when heated to a high temperature. The "glower," as it is called, is composed of one or several short rods of clay-like material. This is first heated by sending the electric current through resistance wire placed directly underneath it and connected in shunt with it. When it gets hot, current begins to pass through it, and is automatically cut off from the resistance coil. The glower produces an intensely bright and white light although it does not itself exceed the temperature of 1742 degrees.

Electric installations are now so carefully constructed that fires from poor insulation are very rare. Less than one fire in three hundred appears to be traceable to that cause.

30. Electric Welding.—Nothing is more common in electrical matters than heat produced by poor contacts. In this laboratory are two chandeliers, each controlled by a wall switch. After the current has been on the chandeliers for half an hour you will always find one of those wall switches warm, while the other is not perceptibly warmer than other objects in the room. The explanation is that there is poor contact in one of them. When two metal conductors touch one another at a mere point the electric current, in passing from one of these conductors to the other across such a narrow bridge, meets resistance and develops heat—sometimes heat enough to fuse the point, and either break the contact, or, what is more likely, start a minute arc at that point. In some cases this makes the apparatus dangerously hot, and in other cases it bridges the gap with a broader and better contact—a true electric weld. Electric welding is applied to everything, from chicken fence to railway rails. Enormously large currents are used for the purpose, in some cases as high as 50,000 amperes being employed. The rails of railroads are welded end to end by a current of several thousand amperes sent through the joint by perhaps two or three volts. The joint heats and fuses together merely because the poor contact offers resistance to this enormous current.

IX

LIGHTING A SUMMER CAMP BY ELECTRICITY

When I did arrive I found they had elaborate schemes indeed. The first floor of the mill had been partitioned off into rooms, as shown in diagram (Fig. 107), a, b, c and d being bedrooms; e was a wash room, the like of which has never been seen before. It had not occurred to me that the mill pond m, which came to the very corner of the building, would furnish the boys a complete system of city water-works. At g, in the corner of this room, they had cut a hole in the floor and nailed slats across upon the under side of the timbers, making a depressed floor for a shower bath. This was directly over a stream of water which issued from the mill pond. Hanging from the ceiling over this spot was the nozzle of a garden hose. The other end of this hose ran into the mill pond. The nozzle was capable of delivering either a stream or a shower, according to which way it was twisted in its socket. It was also capable of shutting off entirely the flow of water. The boys asked me to hold my hand in the shower, and to my astonishment it was warm. "What, pray, is your heating system?" I inquired. They invited me to go and see. Moored outside in the mill pond at the corner of the building was our motor boat, which the boys were allowed to use freely and which they understood as well as any one.

I had hastened the bath, because it was already dusk and I had no candle at the mill, but suddenly the room lighted up as if by magic. I saw then what had before escaped my notice, a miniature electric lamp, six-volt, two-candle-power, tungsten, such as are used for tail lights on automobiles. Since tungsten requires about 1.25 watts per candle-power it was a 2.5-watts lamp, and since it was adapted to six volts it would take about four tenths of an ampere.

6 volts × .4 ampere = 2.4 watts. The little wire filament looked to be about 1.5 inches long. Its resistance must have been 15 ohms.

6 volts/15 ohms = .4 ampere.

A battery of five cells was used to furnish electric current for the lamp. Lamps were installed in the bedrooms also and were not intended to be used more than half an hour at a time. Dry battery cells are excellent for this purpose, and for so small a current the cheapest dry cells are as good as the more expensive ones. These cost fifteen cents a cell. They were connected by short pieces of bare copper wire; No. 18 "in series," as shown in Fig. 109. A wire ran from the central (carbon) binding post of one cell to the marginal (zinc) binding post of the next cell. This battery was placed on a shelf in a convenient place. A bare copper wire, No. 18, was attached to the carbon post at one end of the battery and another to the zinc post at the other end of the battery, and these two wires ran to all the rooms where lamps were placed. The wires were fastened up on the walls by staples, taking care that they should nowhere come in contact with each other and "short circuit" the battery. Whenever it was necessary for one wire to cross another, small pieces of pasteboard were tacked up to prevent their touching each other. The lamps L (Fig. 109) were connected to these wires "in parallel." They cost forty cents apiece, and the miniature sockets, into which they were screwed, cost five cents each. One of these sockets was screwed to the side of the door casing in each bedroom. Wires were attached to the line wires, simply by twisting them together. One of these came down to one side of the socket and the other came to the other side of the socket through a switch, s, made of a strip of sheet zinc. The cost of the entire installation was as follows:

Suppose each lamp is used thirty minutes a day for 100 days, making a total of fifty hours. There are five lamps, making a total of 250 lamp hours. Each lamp takes .4 of an ampere, making a total of 100 ampere hours. The lamps are operated at six volts, making a total of 600 watt hours.

This amount of electrical energy would cost six cents if generated by a dynamo. It is generally stated that electricity costs fifty times as much if generated by battery as by dynamo. In this case the battery actually did serve for the whole season of 100 days and was not exhausted at the end of the season.

Indeed, since that season, the boys have found that battery cells which had been too much exhausted for use on the engine served very well on the lamps. By use the cells lose, not much in voltage, but in the ability to furnish sufficient quantity in amperes to make the hot spark required for igniting the mixture of gasolene and air in an engine cylinder. When they have been discarded for use with the engine they may still furnish the small amount of current required for the lamps—provided not too many lamps are used at one time.

The dynamo current is always surprisingly cheap when compared with that produced by a battery, but, on the other hand, we are never as economical in the use of the dynamo current as we are with that of the battery.

If all five of the lamps in the above equipment were lighted at the same time and kept burning for half an hour, the battery would run down rather badly and would not fully recover. But if one only is used at a time and for not more than thirty minutes, or if more than one is used at a time and for a proportionately shorter period, the battery will receive no damage.

Dry battery cells may be purchased for either twenty-five cents or fifteen cents each. The chief difference is that the former are capable of giving larger current than the latter, when working against very small resistance. For example, the former may give twenty to twenty-five amperes on a short circuit, that is, connected directly with the ammeter without other resistance, while the latter may give not more than six to ten amperes under similar conditions. For most purposes, other than igniting gasolene engines, in which dry cells are used, an exceedingly small current is required. The electric bell, for example, may not require more than .2 of an ampere and that intermittently. Now it is found by experience that the dry cells which are only capable of furnishing on short circuit six to ten amperes will last quite as long in bell work as one which may give on short circuit twenty to twenty-five amperes. Hence it is good economy to buy them.

"What a fine sitting room you have here! (Fig. 107, f.) When do you expect to fit it up?" said I. Instantly reminding myself, however, that boys do not want a sitting room, I inquired what they intended to use this fine, large room for. They told me that they had plans for making a machine shop out of that. The idea had been suggested by a counter shaft which still hung from the ceiling, and they had discovered that the old mill wheel would still roll over if the penstock were repaired. I replied that I would see what could be done about that sometime.

On the next day matters concerning the motor boat engaged our attention.

X

HOW ELECTRICITY FEELS

All this would not have given us the slightest aggravation if we could only have found out what was the matter and what it was we finally did to correct it. But this we shall probably never know, and hence we are worshippers of the motor boat while we continue to distrust it and complain of it.

While the boat was running one of the boys noticed that a binding post at the end of one of the spark plugs seemed to be loose. He inadvertently put out his hand to tighten it and received a terrific shock. This raised the question among the boys, why one gets a shock from some of the binding posts in the electrical equipment but not from others. I suggested that we run in and call at the house to get my portable measuring instrument (Fig. 110) and a little lunch, and then go up to the upper end of the lake and take our time in examining the electrical equipment of the boat.

The engine had two cylinders. There were two batteries—one for each cylinder. Each battery consisted of five dry cells like the one represented in Fig. 111.

"Now, why don't I feel the electricity when I touch the binding posts of this dry cell?" inquired one of the boys as he handled one of the cells which we had taken out. "Well, I'll give you two reasons why do you not feel it," said I. "First, because you were touching only one binding post at a time. You must touch both of the binding posts of the battery cell at the same time, so that the electric current may pass from one post to the other through your body. Second, even when you do touch both binding posts at the same time you feel no current, simply because you offered probably about 100,000 ohms of resistance to the passage of the current and inasmuch as the one cell exerts only 1.5 volts of pressure, it could send only about .0000015 of an ampere through you. This you cannot feel.

"I now connect my instrument as a volt meter between the binding posts of the cell and you see it indicates 1.5 volts, and when I connect it for an instant as an ammeter you see it indicates twenty amperes. That is twice as much as they use for executing criminals by electricity. So you see if you could reduce your resistance sufficiently this one battery cell might kill you. Some people have less resistance than others. The resistance of the body is chiefly in the outer skin. If one's hands are dry and his skin has been made tough and horny by hard work, he has many times the resistance of one whose hands are moist and whose skin is thin and tender.

"Suppose we select the tip of the tongue as the portion of the body which will offer the least resistance and will be most sensitive to slight electric currents. Let us then connect one dry cell with the ammeter and place the tip of the tongue between the bare ends of the wire at T (Fig. 112).

"I have connected the ammeter so that it will indicate thousandths of an ampere, and you see that the needle moves only slightly. We cannot call it more than .001 ampere." Each boy in turn tried sending the current through his tongue and each tried to tell how it felt. One said it tingled, another said it felt warm, another said it tasted sour and the other said he did not feel or taste anything. "Well," I said, "whether you feel anything or not one-thousandth of an ampere is passing through your tongue and you are offering fifteen hundred ohms of resistance.

(1.5 volts)/(1500 ohms) = .001 ampere

"Your hand offers nearly seventy times as much resistance as your tongue. Suppose we try increasing the voltage, or pressure, of our electric current. We will connect in series the ten cells, making a battery which you see by the volt meter gives fifteen volts of pressure. We now find that having ten times the pressure it sends ten times as much current as formerly through the tongue."

(15 volts)/(1500 ohms) = .01 ampere

Each one now testified that the battery sent all the current he cared to take through his tongue. If they send one thousand times as much as that through a criminal no wonder it kills him. It produces a twitch when the contact is first made, afterward a decided sensation of warmth and acid taste.

If we should increase the voltage tenfold more, say the 110-volt dynamo current (direct current), and touch the bare conductors with our hands, the ammeter would indicate about .001 ampere. That is, although this current has about seventy times as much push, or voltage, as a dry cell, no more electricity passes through the fingers than did through the tongue in the preceding experiment with one cell. The fingers offer so much greater resistance.

By wetting the fingers and pressing them firmly upon the bare wires, we may make the ammeter read .01, that is, we may increase the current tenfold by reducing the resistance to one tenth. But there is nothing disagreeable about the feeling. If the same experiment is tried with the 110-volt alternating current, although the quantity of current which passes through the fingers is the same as before, the tingling is more perceptible than in the case of the direct current. If we join together seventy-five dry cells, giving a voltage of 112, and press the bare wires with our wet fingers, the ammeter will indicate .01, but there is no tingling sensation, merely a slight warmth. The battery current, being continuous, causes no twitching of the muscles while the contact is closed. The direct current dynamo furnishes a slightly pulsating current. Hence, one may tell by the feeling whether an electric current comes from a battery or a direct current dynamo. The alternating-current dynamo gives a surging of electricity back and forth in the wires, and this may be distinguished from the direct current by its feeling; when, however, the number of alternations per second is increased very greatly, one may receive through the body considerable quantities of electricity without feeling it. With a very high frequency current one may put himself in circuit and light a 16-candle-power lamp without any disagreeable sensation.

The outer skin is our chief insulation. If it is dry and well toughened by work it offers a resistance of over 100,000 ohms upon gentle contact. A wounded spot, or places like the tongue with moist, thin skin, may offer a resistance as low as 500 ohms. If one has a pin prick or a splinter in his hand which he cannot locate, he may hold one bare wire of a 110-volt alternating circuit in one hand and move the other bare wire about on the suspected region, and know when it reaches the spot by a tingling sensation.