Fig. 69 shows a diagram of the Tesla arrangement with a Ruhmkorff coil. The terminals of the secondary coil of the Ruhmkorff coil _I_ terminate at the condensers _C C_. Bridged across the wires before they reach the condensers is the discharger _D_. The second terminals of the condensers are led through the split primary of the disruptive coil, terminating at the points _B B_ of the second discharger. The secondary of the disruptive coil is either outside or inside the primary coil. The condensers are of special design, being small, but of high insulation. They each consist of two plates of metal a few inches square immersed in oil and arranged so they can be brought nearer together or further apart, as necessary. Within limits, the smaller these plates are the more frequent will be the oscillations of their discharge. They also fulfil another purpose, they help nullify the high self-induction of the secondary coil by adding capacity to it.
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.
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.