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Electrophysiology.—Some further particulars are to hand with regard to the experiments by M. Leduc on alternating electrostatic currents.
THE ELECTRICAL ENGINEER, JULY 28, 1893. 77
When an electrostatic machine is worked and a series of sparks apparently continuous
pass between the knobs, the conductors are, as is well known, subject to variations of potential—in fact, to electric vibration. These vibrations are utilised by M. Leduc to produce other alternating or vibratory currents. If a Leyden jar is suspended by a rod from its inner coating to each of the conductors of the machine, and the external coatings of the two jars are connected by a high-resistance circuit, the internal coatings become part of the conductors. Electric vibrations of reverse nature take place at the same instant of time on the two inner coatings, one rising as the other falls, and the vibration is communicated to the outer coatings, so that on the circuit connecting them a vibratory current is set up. The resistance between the outer coatings should be such that the total capacity of the Leyden jars is proportioned to the output of the machine, If, for instance, the two coatings are connected by a circuit of negligible resistance, the conductors (of which the coatings form part) having too large a capacity, the sparks become intermittent, and a vibratory current in the true sense of the word is not obtained. But with a high resistance between the external coating, oscillatory discharges could not be produced between these coatings. The experiments were carried out with a Wimshurst machine having two glass plates 2ft. diameter. The vibratory currents obtained by the means of electrostatic machines have properties similar to but not identical with those used by Mr. Tesla. Conductors carrying these currents become luminous, and, under good experimental conditions, give from their whole surface brushlike discharges differing much in intensity and other characteristics from those of electrostatic electricity. By varying that form of the conductors and by bringing other conductors near, very varying and very beautiful luminous effects are produced. The best arrangement to carry out this experiment consists in putting one of the external coatings to earth and connecting the other with a hanging chain in contact by a point with the base of the machine. If the hanging chain of the preceding experiment is covered with an indiarubber tube, and this tube is taken in the hand, it becomes luminous as if phosphorescent by the production of short brush discharges, in the form of an aura between the hand and the tube ; the sensation is not painful. If a Tesla tube is brought near to the free conductor, it lights up at a considerable distance away, and if the balls of the exciters are separated so as to suppress the alternations while increasing the tension, the tube becomes dark, and must be brought much nearer to light up again. An incandescent lamp bulb suspended on the free conductor becomes luminous, and its luminosity increases if touched. This lamp and the india- rubber tube itself are both strongly attracted by a conductor in communication with earth—by the hand, for instance. The same experiment made with static electricity gives an attraction followed by a repulsion, while with these vibratory currents only attraction is produced. These high-tension alternating currents obtained with static machines are noticed to have, as we have already mentioned, a great effect on the sensory and motor nerves. One electrode is sufficient. A blunt metallic point is used, mounted on a glass handle, attached to the free chain of one of the external coatings; this point moved over the skin, when passing a motor or sensor nerve, excites the nerve throughout its whole distribution beneath the electrode. The sensation throughout the region covered by the nerve is so plain that it allows the distribution to be mapped out. MM. Pellat, Gariel, and D'Arsonval verified these experiments of M. Leduc. But the experiment which caused the most interest and surprise was the production of muscular contraction at a distance by the aid of currents themselves produced in the human body by induction. The observer being placed between the machine and a live frog, if he moves his hand near to the frog as if to point at it with his forefinger, even at the distance of so much as a yard away, the muscles begin to contract. If the hand is moved nearer or farther away, these movements are inscribed on the myograph by this new kind of galvanoscope.
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