The Testing of a Coil for Polarity.
This is often necessary, and may be done in a variety of ways. When the coil is working, and sparks be passed between fine wires mounted on the discharger, the positive wire tip will be cold, whereas the negative end will be quite hot. In vacuo, the positive shows a purple red when the negative glows with a bluish violet. The decomposition of water, which consists of oxygen and hydrogen in the formula H2O, is readily accomplished by the secondary current, and the greatest volume of gas (hydrogen) will be evolved at the negative pole. For ready reference a summary of these facts is given below:
| Positive | Negative |
|---|---|
| Cold wire, | Hot wire, |
| Anode, | Cathode, |
| + sign, | - sign, |
| Purple red, | Bluish violet, |
| Zinc plate, | Carbon plate, |
| (Carbon) pole, | Zinc pole, |
| Oxygen gas. | Hydrogen gas. |
Although it is customary to use bundles of fine, soft iron wire for coil cores, very excellent results have been obtained with cores made up of soft iron filings. These filings should be tightly packed in the core tube and have a soft iron head at the contact breaker end. Filings demagnetize very quickly and prevent the formation of destructive eddy currents, which have been previously discussed (Chapter I.).
Modern practice tends towards a lengthening of the core and primary, in some cases fully 20 per cent of the core length projects from each end of the coil. One result must be as in electromagnets, the longer the core, the longer it takes to magnetize or demagnetize. But even here it is a matter of individual construction.
The common practice is to make coils to be in a horizontal position; there is no reason why they cannot be made to stand on end. In fact, this position to an extent takes off some of the strain on the primary. It is mostly a matter of choice or convenience.
As to the possible output of an induction coil, it depends upon design and construction; but S. P. Thompson gives the following law in his work on Electricity and Magnetism: The electromotive force generated in the secondary circuit is to that employed in the primary nearly in the same proportion as the relative turns of the two coils.[1]
[1] We do not attempt to reconcile this quotation with the enormous estimates of spark potential.
In selecting a Ruhmkorff coil, it must be remembered that the rating in spark length is subject to question. Supposing two similar coils be operated, one with a rapid vibrator and the other with a slow vibrator, other things being equal, the slow vibrator will give the greatest spark length. Again, the appearance of the spark is of vast importance. Although two coils might be sparking across the same length air-gap, the one giving the whitest and thickest continuous succession of sparks is the better. Fig. 13 shows a reproduction from a photograph of a spark 32 inches long, generated by the coil shown on the frontispiece.
Fig. 13.
It is easy to take a coil, and by snapping the vibrator contacts together a few times a spark of thin bluish character will jump across a gap, of length far exceeding the spark gap when vibrator is working at normal speed. But this spark only passes at irregular intervals, seemingly gathering strength for its forced leap. It must not be considered in rating the coil.
In winding primary coils it is proposed to reduce the self-induction or inductance of its adjacent coils by means of similar methods used in winding electromagnets. The primary winding, instead of being composed of a number of turns of one large wire, is made up of a multiple winding of small wires, aggregating the conductivity of the large wire. This materially reduces sparking at the contact breaker, and certainly allows of a closer bedding of wire nearer the core, also giving a greater percentage of ampere turns. Another scheme which uses the Dessauer contact breaker provides two separate primary windings, opening one when the other closes. Such schemes as these come well within the scope of the experimenter, and it is highly possible that valuable improvements will be made in coil design during the coming years.