Adjustable Condensers.
In operating large coils, it is convenient to be able to vary the capacity of the condenser on the primary circuit. To make an adjustable condenser presents no more difficulty than a non-adjustable one, simply more labor. For example, the large condenser used with the 6-inch spark coil might be divided into four sections, containing 2000 square inches, 500 square inches, 300 square inches, and 200 square inches of surface (see Fig. 34). Wires leading from the ends of the foil sheets C C are to be brought to the brass plates G G. The brass rods B B are connected by binding posts to the coil, each strip being well insulated from its neighbor. Any combination is possible by the insertion of brass plugs in holes drilled between the strips. The plugs must be fully large enough to make good contact on each of the two strips between which they are inserted, and should be turned taper. With the largest coils the condenser and contact breaker are generally mounted separately, and are fully adjustable.
Fig. 34.
| Specific Inductive Capacity. | |
|---|---|
| Dry air | 1.000 |
| Sulphur | 2.590 |
| Hard rubber | 2.290 |
| Paraffin | 1.996 |
| Shellac | 2.750 |
| Kerosene | 2.225 |
| Paraffin oil | 2.710 |
| Castor oil | 4.962 |
| Olive oil | 3.575 |
Condensers made with dielectric of high inductive capacity (insulation being equal) will retain greater charge than those made with dielectrics of low inductive capacity. Thus, one made with shellac would be nearly half as great again as with paraffin.
Capacity of a condenser increases with area of foil surface, with diminished distance between foil plates and with increase of insulation.
CHAPTER V.
EXPERIMENTS.
The luminous effects that can be obtained by means of a Ruhmkorff coil are exceedingly beautiful and instructive. The simplest experiment of this nature is the production of the spark consequent on the approximation of the electrodes attached to the secondary coil. This spark can be varied in both length, intensity, or shape by the form and nature of the substances between which it is permitted to pass. Attach to each end of the discharger a fine steel needle, and bring them together until the spark jumps from one to the other. A long thin snapping spark will pass, which, however, appears to be trying to take any but a straight path across the air gap. The peculiar crookedness of this, as in a lightning flash, is credited to the fact of particles of matter floating in the air conducting the current better than the pure air. The curious odor noticed in these discharges, as, in fact, in the working of all high-tension apparatus, is ozone—O3, triatomic oxygen. This gas, so noticeable after a thunderstorm, has a powerful effect on the mucous membranes of the throat and nasal passages, and must be inhaled with caution. It is being used by the medical profession for the destruction of germs and for general therapeutic service.
Substitute pieces of fine iron wire for the needles, and bring the ends together about one quarter the distance through which the normal spark will pass. The spark will be found to have changed its appearance, now being thick and redder, or, rather, of a deep yellow, and to possess vast heating qualities.
The iron wire will melt at one electrode, and if the other be examined it will be perceived that it has not even become warm. The cold wire will be the one connected to the positive pole of the coil.
Connecting the poles together with a piece of very fine iron wire will result in the deflagration of the wire in a vivid light.
The short thick spark is termed the calorific spark, and believed to possess its yellow color from the combustion of the sodium in the air. This spark will easily ignite a piece of paper held in its path.
Take a sheet of hard rubber and breathe on its surface; lay a wire from each pole of the secondary to points on the sheet, about twice as far apart as the spark would pass over in the air. The electric current will strive to complete its circuit; streams of violet light forming a perfect network will issue from each pole, until, provided the rubber is sufficiently damp, they will unite in a spark far exceeding its normal length in the air. It is curious to watch how the streams branch out from these two points, and how persistently they strive to meet each other. Scatter some finely powdered carbon on this sheet (crushed lead-pencil or electric light carbon is good material). The points may now be removed to still further distant places, and yet the current will work across. Each particle of carbon seems to be provided with innumerable scintillating diamonds, so sparkling is this effect.
Hard rubber is not absolutely necessary for these experiments; glass will do, but the black background of the rubber intensifies the luminosity of the discharges. Take a teaspoonful of powdered carbon and scatter it between the points on the rubber, so that the spark can find a ready path, evidenced by but little visible light. It will be seen that this powder is blown away from one electrode after a few minutes, leaving the latter in the centre of a clear space, but at the other electrode not much disturbed.
Bring the points so close to one another that the spark becomes short and fat; soon the carbon will commence to burn, forming a veritable arc light. Take two pointed lead-pencils and wrap a few turns of wire from the electrodes round the blunt ends of them; bring the pointed ends together, and an arc will soon be established; but at various points where the wire is wrapped the current will burn through the wood, and a number of incandescent points will ensue.
In these experiments on the rubber sheet it will be noticed that the spark acts as it does in the air, inasmuch as it does not take a direct path, but jumps in an irregular track from point to point.
If two small metal balls be substituted (Fig. 35) for the points between which the sparks be passing, it will be noted that the sparks do not pass through so great an air gap as before, or even as rapidly.
The spark between two balls is much noisier than that passing between points, and if the balls be of about 1 inch in diameter, a curious effect ensues on the passage of the current (Fig. 36). This effect has been likened to a stream of water issuing from a horizontal nozzle into a cavity when the nozzle is moved up and down slowly in the space of a few inches.
Fig. 35.
Fig. 36.