Series.
The smaller the amount of surface the less will be the capacity, but the quicker the discharge. The apparatus heretofore mentioned has had the alternate plates connected together in two series, presenting a large surface and rendering a large amount of current. A condenser so made will have a low voltage or potential, but is not so liable to leakage as one made to render a high potential. The multiple condenser of a large capacity will hardly discharge and spark over an air gap requiring a contact of the two electrodes. But a smaller one, consisting only of a single pair of small plates, will spark across quite a considerable air gap.
A number of charged condensers may be put in series, and the resultant potential thereby increased. Cut a number of pieces of paper of the desired size, say 6 inches square, and a number of sheets of foil 3 inches square. Round off the corners of the foil and build up first a sheet of paper, then a sheet of foil in its centre, then another paper and another foil sheet, and so on. There is to be no connection from sheet to sheet, only the inductive action of one on its neighbor. The foil must be considerably smaller than the paper in this construction, owing to the greater tendency to discharge round the edges of the sheets, owing to the greater potential of the current.
When the requisite number of sheets have been built up, leave a sheet of foil top and bottom for connection, tie between two pieces of stout card or board, and immerse in the molten paraffin. When thoroughly soaked, remove and put under pressure until cold. It will be found undesirable to make these with more than a dozen pairs of sheets, but to make a number of blocks of that number for ready service.
Fig. 32.
Fig. 32 shows the arrangement of the apparatus to charge a Leyden jar, the plate form being connected in a similar manner. The jar is stood upon an insulating support—a dry tumbler will answer—with the ball B connected to one pole of the coil. From the outside tinfoil coating T a wire runs to the discharger D D, which is in circuit with the secondary coil, S. The discharger balls D D are carefully approximated until the spark just passes, this latter point being of great importance. Were the discharger balls too near the spark would probably pierce the dielectric of the condenser, therefore the balls should be carefully brought near to each other until the exact distance is found. Even if the insulation of the condenser were not pierced, yet a path would probably be opened through which some succeeding discharge would pass, and ruin the instrument.
Another method of charging is to leave an air gap at B; then there is not much liability of the condenser discharging back through the coil—an undesirable event, as it would most likely perforate the insulation of the coil.
Fig. 33.
In designing or using any apparatus intended to hold a charge of high potential, it must be kept in mind how readily points or sharp edges serve to allow the current to pass off—we might almost say evaporate. Given two bodies, one a globe and the other a rectangular block, each well insulated from the earth or any other large body, and the globe would be found to hold its charge long after the block had dissipated all trace of the charge given to it. Therefore round off every edge and angle, projection or point.
In making handles, supports, or any work requiring an intervening high insulation, hard rubber is preferable to glass where there is liability to moisture. When the apparatus is as shown in Fig. 33, the condenser is alternately charged and discharged with a loud noise, the vivid sparks passing across the discharger balls D D possessing great deflagratory powers.
In experimenting with a Ruhmkorff coil it is not advisable to leave the instrument working while the secondary terminals are beyond sparking distance, as there is a great strain on the secondary insulation. Nor is it wise to use only one electrode in an experiment, unless the other is connected to some apparatus of an approximate capacity to that at the other, for the foregoing reason.