If we now consider the effects of alternating currents, we find that we have another factor to deal with. The general principles are exactly the same, but inasmuch as the reversal shock is stronger than the closure or opening shocks, other things being equal, we are likely to receive a stronger shock from a current of the same force, and in addition to this, as in alternating machines the reversals occur with considerable rapidity, the person who becomes connected with this circuit receives a number of strong shocks within a short space of time. This is a much more serious matter than to permit a current of equal strength to flow through the body without change.
—The effect of this form of electricity on the human body is first stimulating and then tetanizing to the muscles. It consists in a very rapidly interrupted current, the shocks being at times so frequent that they are not singly perceptible. There is probably also a distinct difference in the action of this current from that of the galvanic current aside from its rapid interruption. This is not, however, of so defined a character as to enable us at the present time to distinguish in man the results of severe injuries and deaths caused by this form from those caused by other strong currents. Practically this form of current is but little used, except in medical batteries and for the purposes of experimentation in laboratories.
Static electricity has, so far as we know, rarely or never caused serious injuries or death. The sparks produced in this way have sometimes caused burns, and it is conceivable that a strong electric current produced in this way might be dangerous. The symptoms could not be distinguished from those caused by other forms of electricity.
Summary.—The greatest source of danger from electric currents is the shock produced by them. In ordinary constant or continuous currents this is produced only at the moment of the opening and the closure of the circuit. In alternating currents a shock is also produced at each reversal of the machine. Faradic and static currents are rarely or never used mechanically or in the arts.
Resistance.—The resistance of the human body to electric currents has been very variously estimated.
The reasons for these variations are: (1) that the different tissues present different resistances; (2) that the resistance in the same tissue varies greatly under different circumstances.
The tissue which offers the greatest resistance and also practically the greatest variation is the skin, or, more properly speaking, the epidermis. The resistance of this is many times as great as that of the rest of the body, and when perfectly dry it is impervious to currents of great strength. Witz states that in using a Ruhmkorf coil with an estimated force of 250,000 volts in Guinea-pigs and rabbits, it is advisable to cut through the skin in order to apply the electrodes directly to the flesh, or, at least, to wet the skin thoroughly, otherwise the shock caused by the full strength of the battery (six jars charged from the coil) would not cause death. Various animals offer rates of resistance which vary somewhat apparently according to the nature of the animal, but are probably largely dependent on the conducting power of its tissues, that is, of its skin. The variations between the resistance of similar animals, according to the condition of the skin at the time of the experiment, are much greater than those which are found between animals of different species under similar conditions, or which are referable to specific susceptibility. Mr. Harold P. Brown testified in the Kemmler case (Court of Appeals, State of New York—State of New York ex rel. William Kemmler against Charles F. Durston, agent and warden) that he had in the course of his experiments seen a horse weighing 1,320 pounds, with a resistance of 11,000 ohms, killed by an alternating current at 700 volts.
The resistance of the different cutaneous surfaces of the human body as measured by Jolly in Siemens’ units was from 400,000 down to 15,000 in the male and to 8,000 in the female (Siemens’ unit is to the ohm as 1.06 to 1.00).
Tschirfew and Watteville made the resistance from 80,000 to 3,000 ohms.
Experiments made at the Edison Phonograph Factory and Edison Laboratory in July, 1889, on 259 males between the ages of eleven and fifty-one, showed a resistance, measured between the hands immersed to the wrists in a solution of caustic potash independent of polarization, averaging 986 ohms and varying from 1,970 to 550 ohms.