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.
