Part 16

"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.

One may touch lightly the 220-volt direct current and scarcely note any difference between this and the 110-volt direct current, because one is not very sensitive to the difference between .001 ampere and .002 ampere passing through his body.

(100 volts)/(100,000 ohms) = .001 ampere, and (200 volts)/(100,000 ohms) = .002 amperes

Physicians treat certain ailments by the use of the electric current. For this purpose they invariably use a pulsating or alternating current and reduce the resistance by using metal handles and wet sponges for contact with the skin, but even so a very small amount of current passes. The moderate twitching of the muscles seems to be the end sought.

Men who are supposed to be killed by electric shocks often die from other causes. A man perching upon an electric light pole, repairing wires, may come in contact with a wire charged, say, to 2000 volts. He may receive a shock which throws him in an unconscious condition across another live wire which burns its way into his flesh, or he may fall to the ground and be killed by the fall. A workman may hold a tool so as to short circuit a current through it, making it red hot in his hands. So many men who have been shocked into unconsciousness by high voltage currents have recovered consciousness later that we cannot say how much current is required to kill a man. For the execution of criminals 1800 to 2000 volts are used, and by special metal contacts ten to fourteen amperes are forced through the body.

The first execution of a criminal by electricity was performed in Sing Sing Prison, New York State, in 1890. There was at that time a hot controversy among experts over the question whether death, or merely unconsciousness, could be produced by electricity. To be on the safe side the legislature passed a law requiring that the electrocution of a criminal should be followed immediately by the dissection of his body. Only six states out of forty-nine have thus far adopted that method of capital punishment, five have abolished capital punishment, and thirty-eight still prefer hanging to electrocution. But it should be remembered that it is amperes, not volts, that kill. One often hears the meaningless expression, "he received 2000 volts into his body." The volts indicate the pressure, analogous to pounds per square inch of water pressure. Amperes of electricity are analogous to gallons of water. It is possible to have exceedingly high voltage of electricity without amperes enough to do damage. When one holds his finger near to a rapidly moving leather belt and a stream of sparks passes between the finger and the belt, the voltage may be 50,000 or even 100,000, but the quantity in amperes is too small to do any damage or even produce much sensation. A similar thing is true when one produces sparks by rubbing a cat's back, or lights the gas by a spark produced by rubbing the feet upon a carpet. Such sparks are miniature lightning discharges. The real lightning does damage because it furnishes quantity, measurable in amperes, as well as extremely high volts of pressure.

At this point I was reminded by the boy who had received a shock from the engine that morning that he had touched only one binding post. How then had he closed a circuit through his body, and how could he receive such a terrible shock when there were only a few battery cells to produce the electric current. I replied that he had the distinction of having encountered about a 5000-volt current. In the language of the newspapers he might say, Took 5000 volts and still live. We must next proceed to show how he really did close the circuit and how the spark coil enables a battery of a few dry cells to produce exceedingly high voltages.

THE ELECTRICAL SPARKING EQUIPMENT FOR A GASOLENE ENGINE

Under the shade of a great sugar maple, with Millville Lake spread before us, we took apart and examined the entire equipment for producing the electric sparks to explode the mixture of gasolene and air in the cylinders of our motor boat. The engine has two cylinders. For each cylinder there is a separate battery and spark coil. Inasmuch as the electrical outfit is duplicated for each cylinder it will be necessary for us to consider the case of one cylinder only.

When this engine is running, 700 explosions per minute are produced in each cylinder. In one-twelfth of a second the following four events take place:

1. The cylinder is swept clear of the products of combustion formed by the last explosion.

2. Four drops of gasolene are vaporized and mixed with one quart of air and pushed into the cylinder by the pressure of the atmosphere.

3. This mixture is compressed by the piston in the cylinder to about one-fifth its original volume.

4. The mixture is heated to its kindling temperature, which is above 2000 degrees. It then burns with a sudden expansion, which drives the piston before it and pushes the crank which is concealed in the lower end of the cylinder half-way around. The crank is attached to the shaft, which carries the fly-wheel upon one end and the propeller wheel upon the other end. The momentum of the moving parts--chiefly that of the fly-wheel--suffices to accomplish the remaining half of the revolution.

That any machine could be devised which could repeat these four events 700 times a minute was unthinkable a few years ago.

The first men who thought that a gasolene engine could be a practical thing were considered visionaries, but now they are found to be more practicable than steam engines. They are so efficient that they compete with the steam engine upon its own ground, and, in addition, they have opened up regions of usefulness which the steam engine can never exploit. So far as we can see, they have a permanent monopoly of the navigation of the air.

It is with the fourth event mentioned above, viz., kindling the explosive mixture, that we are now concerned. The high temperature required for this is obtained by forcing an electrical current against resistance.

Five dry battery cells would very readily heat a short piece of fine wire to a sufficiently high temperature to explode the mixture, but it is impossible to alternately heat and cool a wire twelve times a second. It is too slow an operation. The only other method known at present is to imitate the lightning and force an electric current against the resistance of the air with sufficient power to produce the required heat. This, however, requires an extremely high voltage--at least 5000 volts, and our battery of five cells has not more than seven and a half volts of pressure. The interesting question then is, how does the spark coil enable us to raise the voltage from 7 to 5000.

To help toward an understanding of the matter I took seven small wire nails which I found in the boat--they were sixpenny finishing nails. I then took two or three yards of No. 24 insulated magnet wire, such as is used upon electric bells, etc. I use it more often than any other wire, and always have some about the boat. I fastened one end of this wire to one of the binding posts of a dry cell (Fig. 113), a, and attached branches c and d to it. The other end, b, was left free to act as a switch for closing the circuit by touching it to the remaining binding post.

One boy then touched the bare ends c and d to the tip of his tongue, while I touched repeatedly the binding post with b. There was, of course, no sensation. We now wound a portion of the wire upon the bundle of nails, laying on about fifty turns. (See Fig. 114.) The tongue was now placed at T and b was touched a few times to the free binding post. A very decided shock was felt, not while the end of the wire was resting upon b, but at the instant of touching and again at breaking the connection. The shock was noticeably stronger at the instant of breaking than of making the connection. There was also a spark formed when the connection was broken, which did not appear before the coil was made. We next wound on more of the wire--about fifty more turns (Fig. 115). When now connections were made and broken at b the tongue at T felt a much more decided shock, and a larger spark occurred at b when the circuit was broken. Both the tongue and the spark indicate that the voltage is creeping up very rapidly in this series of experiments. We next connected two cells in series, then three, four, and finally five cells in place of the one. The spark grew larger and "fatter," as the boatmen say, with each addition of a cell. It was not pleasant to use the tongue in the experiment after the number of cells exceeded two. I removed the branch d from the wire b and connected it to the binding post, as shown in Fig. 116. I then removed the crystal from my watch and poured into it a little gasolene. I rubbed the ends of b and d together over this, and when they separated the spark which was produced would not light the gasolene. We had made a coil which produced a spark that looked like a miniature flame, but still was not hot enough to set fire to gasolene vapour. It simply needs more iron in the core and more turns of wire about it. Bringing the ends of the wires together and separating them is somewhat like drawing an arc with the arc light carbons. It requires a vastly higher voltage to make a spark jump across an air gap than it does to lead it across thus.