The discharger tips are preferably metal balls under 1 inch in diameter. Tesla uses various forms of dischargers, but for experimental purposes the two metal balls will answer. They are adjusted when the whole apparatus is working according to the results desired. The mica plates serve to establish an air current up through the gap, making the discharge more abrupt, an air blast being also used at times for the furtherance of this object. The device (Fig. 70) consists of an electro-magnet, C, set with its poles P across the air gap, helping to wipe out the spark, as in a well-known form of lightning arrester. This form, described by Tesla, has the pole pieces P shielded by mica plates M, to prevent the sparks jumping into the magnets. Fig. 70 is an elevation and Fig. 71 a plan of this device.
Fig. 71.
The resonance effects obtained during the operation of a Tesla coil are very marked, and their study may lead to the solution of the problems of communication between distant points without the use of other conducting media than the atmosphere. But the main use to which the Tesla currents have been put is that of artificial illumination. These currents have enabled experimenters to obtain a high luminosity in vacua by the aid of only one conducting wire—in fact, in some cases without the utilization of any conductor than the air. An ordinary incandescent lamp connected to one terminal of the coil will show in a fair degree some of the luminescent phenomena. The brush effects from the terminals of the secondary coil are extremely marked and interesting; but to detail the experiments that can be performed with the Tesla disruptive coil would be an impossibility here. Reference is recommended to the published works of Nikola Tesla, which happily are readily procurable.
These currents of high frequency have of late been turned to account in electrotherapeutics, principally for the stimulation they exert on the nutritive process. They also exert a very great influence on the vasomotor centres, as is evidenced by the reddening of the skin and exudation of perspiration. This result is readily obtainable by placing the patient in connection with one electrode on an insulating stool, and terminating the other electrode at a large metal plate situated a few feet distant; or the patient may be surrounded by a coil of wire in connection with the coil of sufficient diameter, however, to prevent contact.
CHAPTER XIII.
THE "ROENTGEN" RAYS AND RADIOGRAPHY.
Although the remarkable discovery that it was possible by electrical means to depict an image of an object on a photographic sensitized plate, despite the intervention of solid bodies, was first given to the world at large by Professor Roentgen, yet he was undoubtedly led to the results by consideration of the works of previous experimenters in electrical discharges through vacua.
It is not intended here to trace the previous work of Professor Crookes, the inventor of the radiometer, which is actuated by the heat rays of light, nor of Hertz, who found that gold leaf was transparent to rays emanating from certain vacuum tubes carrying a luminous electrical discharge. It is mainly the purpose of these pages to give directions for practical work, and not deal in theories, interesting though they be. At the beginning of X-ray investigation many claims were made which have since been disproven, but the fundamental operations remain the same. A Crookes tube of special design is energized from a coil or similar electrical distributor, and by means of the resultant rays otherwise opaque objects appear partially transparent, their shadows being cast upon the screen of a fluoroscope, or these shadows are allowed to act upon a sensitized photographic plate, and subsequent development reveals outlines or shadowgraphs. The general arrangement of apparatus is shown in Fig. 72. C is a Ruhmkorff coil, giving not less than 2 inches of spark; B the battery operating same; T the modified form of Crookes tube used most generally; X the object under observation; F the fluoroscope or the sensitized photographic plate. The usual precautions are taken to avoid the leakage of current from the secondary wires, the tube T being best mounted in a wooden stand (Fig. 72), and the wire connections brought to it as direct as possible. No condenser, stand, etc., are shown in drawing, to avoid unnecessary complication.