Tesla Currents with Simple Apparatus

Mr. F. Himstedt publishes in Wiedemann's Annalen, vol. 52, page 473, some interesting researches with Tesla currents, produced by means of rather simple apparatus. We ge below a translation of an abstract of this paper as published in the Elektrotechnische Zeitschrift, July 12.

The reason why the researches of Tesla have not been reproduced in Germany is probably due to the fact that the apparatus which he used was not at hand. It is very interesting to note that the author succeeded in reproducing most of the results with exceedingly simple apparatus. He at first used Lecher's arrangement for the production of Hertzian waves, but afterward used still better known apparatus, namely, Leyden jars.

He connected the pole of a powerful Ruhmkorff coil (50 cm. long and 20 cm. diameter, using from 5 to 6 accumulators) with the inner coatings of two moderately large Leyden jars, insulated from each other by being placed on paraffine; he also used a spark micrometer. The outer coatings of the jars were connected with a 4 mm. wire, 150 cm. long and bent in the shape of a U. A 16-volt incandescent lamp connected in parallel to this wire was brought to a white heat by proper adjustment of the micrometer. By using jars 16 cm. in diameter and 42 cm. high, three lamps could be made to glow, hung over each other, one a 65-volt lamp at the bottom, then a 16-volt lamp in the middle and a 2-volt lamp at the top.

If the middle of the glass bulb of such a lamp is filed off it will either not glow or glow very little. Tesla explains this by the different bombardment of the molecules of the glass in the bulb, while the author ascribes the result to the different heat conducting power of the air which entered.

If the incandescent body is a platinum wire of 0.05 to 0.1 mm. diameter, it will oscillate rapidly while glowing, taking the form of a zigzag line, not in a single plane like a stretched string, but in all possible planes, thus representing a model of a ray of natural light. A condition for the success of the experiments is the use of an interrupter which operates with certainty, as also in the use of a proper spark micrometer. He recommends the Foucault interrupter with tough zinc amalgam instead of mercury, covered with machine oil. The spheres of the micrometer are best made of zinc.

A Tesla transformer was constructed as follows: On a glass cylinder of 4 cm. diameter he wound 10 turns of a 4 mm. wire, with a distance between the windings of 1 cm., over which a thick glass cylinder, or better, a hard rubber cylinder of 6 mm. thickness, was placed, and on it were wound 200 windings of a 1 mm. wire. In order that the distance between the windings may be made 1 mm. two similar wires were wound next to each other, after which one of them was again unwound. The transformer was placed horizontally in an earthenware vessel filled with machine oil, and supported on hard rubber, and the secondary winding ended in two metallic balls on hard rubber pillars. The primary winding was connected with the Leyden jars in place of the above mentioned U form wire. When the interrupter is in action, brush discharges will appear at the knobs; if a conductor is placed near them, bright sparks will pass to it. Between two parallel wires connected to these knobs, a bright band of light of three to four metres in length will be formed.

If one pole is connected to. earth and the other to a thin wire 15 to 20 cm. long which hangs freely suspended, it will begin to move and describe a cone-shaped figure which is distinctly visible in the dark on account of the brush discharges which take place from it.

If a person takes hold of one pole of such a coil, bundles of rays can be drawn out from any part of his body near which a conductor is approached.If the person is standing on a metallic plate a prickling sensation will be produced in the feet; in the dark, discharges will be seen to pass out of the shoes. If several persons are connected together the first taking hold of one pole of the transformer, while the last holds a Geissler tube with or without electrodes, this tube will be brightly illuminated, although none of the persons will feel the discharges.

If one pole is connected to earth and the other to a large metal sphere, 60 cm. in diameter, Geissler tubes will be brightly illuminated at distances of from three to four metres from this sphere.

An incandescent lamp connected with the pole which is not connected to earth shows only the pale light of a Geissler tube. But if a large metallic disc is connected to the glass bulb, the lamp will glow brightly. The filament will, in that case, oscillate so that it soon breaks. If a conductor be approached to the point of the lamp this point will be immediately pierced by the current and a continual bright stream of sparks will pass through the opening, and the filament will at once begin to glow until it is consumed.

In the second part of his paper the author states that a Tesla coil is quite different in its actions from an ordinary induction coil. A Geissler tube connected in any way with the pole of a Tesla coil shows cathode light at both electrodes, an anode light in the middle. A commutation of the primary current or a reversal of the pole has no effect. An electroscope is always charged positively when it is approached with a pole terminating in a point. Both poles produce only positive Lichtenberg figures, never negative.

Researches with different gases show that this preponderance of the positive discharges from the pole of a Tesla coil depends on the surrounding medium. Air or oxygen promote positive discharges; hydrogen, illuminating gas, nitrogen, carbonic acid gas and ammonia, promote negative discharges.

If two points are connected with one pole, one in air and the other in hydrogen, positive and negative electricity will be discharged simultaneously from the same pole, depending on the surrounding gas. If one of the points is in oxygen, and if gradually more and more nitrogen is added, an electroscope opposite the point will at first show a moderate positive charge; with increasing quantities of nitrogen this will increase; when the mixture corresponds to that of air the positive charge will reach a maximum; if the nitrogen is in excess the charge will be reversed and will become negative. The process is similar if it is begun with nitrogen and the oxygen gradually added. With an ordinary induction coil the charge of an electroscope will be reversed when the primary current is reversed.