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.
Photograph by Helen W. Cooke
Feeling Electricity
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.