The modern American type of Ruhmkorff transformer, or induction coil, is one of the most important pieces of electrical apparatus to be found in an experimental laboratory. It differs so materially in efficiency from the old fashion types of foreign construction, that to construct a coil along the lines of a decade ago, would be at a considerable waste of time and material in the winding.

The one to six inch spark coils, as constructed by the makers of today, are the result of delicate calculations made in connection with the construction of thousands of coils to be used in "wireless" work, and gas engine operation, each point of detail being perfected by comparative experiment in the mutual relation of one part of the coil to the other. It is obvious that in these numerous tests, many old-time theories regarding transformation have been eliminated, partly because of proven inaccuracies, and also because commercial competition and the increasing demand for coils to meet certain requirements have brought forth winding machines for bobbins of standard dimensions and specifications securing the greatest efficiency with a miminum of time and labor.

The fundamental principles of the conversion of the electric current are now so well known that an extended description of the simpler forms will not here be given. Old time builders theoretically calculated, disregarding wire resistance and core loss, (hysteresis) an objectionable property which the iron core has, when in action, of resisting the magnetizing current, that the Voltage times Amperage in the primary coil equal V times A in the secondary coil. From this it was deduced that any variation in the number of turns in either winding produced a change in the relation of Volts and Amperes in the primary to that of the secondary coil, all of which is eminently true. But much, however, has been learned by recent work regarding the adjustment of the primary to the work required, the insulation of the windings, and the proper disposition of the secondary bobbins upon the primary, and the construction of the current break or "interrupter".

This modern research has opened new possibilities, and today the coil makers have no difficulty in constructing sectioned coils to give any desired spark, (20 inches if necessary), wound in perfect layers with suitably prepared insulation between each layer.

Obviously, between contiguous "turns" in the secondary coil there is practically no potental difference and little tendency to discharge. With sufficient layers of dielectric substance between each layer of wire the chance of injury to the windings is very slight, this insulation being sufficient to withstand any potential which can be derived from the magnetic influnce of the primary coil.

When the secondary is formed of independent sections, which are wound in such directions as to permit of opposite terminals being connected together, the potential near the primary becomes little more than zero; the coil is then able to withstand an enormous strain, and old time difficulties of secondary discharging into the primary are no longer troublesome.

In the construction of secondaries, the amateur becomes puzzled in his calculations, both as to the amount of fine wire to be used to obtain a certain spark, and the proper disposition of this wire in turns upon the primary, for it is the turns which count. Miles of magnet wire indiscreetly placed in the secondary, would oppose the influences of the magnetic field, and not-withstanding how efficiently the rest of the coil might be constructed, the resulting spark would not come up to the expected length and "fatness " ; in fact there are in use today, commercially, many coils wound in such a manner that one third of the wire is worthless, or in other words, had this wire been properly adapted to the primary the results would have been one third greater.

In considering a practical coil, we first enter into the theories of a most unique force of nature, - magnetism. We learn that a core of annealed iron wires, of a prescribed length and diameter, has a limit in magnetic saturation, and when that point is reached we may estimate what may be expected from a secondary bobbin properly placed upon the core. For this core, when magnetised, will influence a certain cross-section in a secondary winding, and while the ratio of cross-section of the core to that of the secondary may vary in the plotting of magnetic curves for coils of low and high frequencies, a well accepted rule is to limit the secondary diameter to about twice that of the primary, these calculations being determined by tests made in the construction of many small coils now in general use.

In "filling" and "emptying" a primary winding with electric current by means of a vibrator, or interrupter, we note a certain relation between capacity and frequency. We note that a large secondary containing many turns of wire is often poorly adapted to high frequency, while a smaller one having less turns, sacrifices potential and gains quicker discharge. We are led to appreciate, therefore, that the best interrupter for the amateur's first experiments is one adjustable within reasonable limits, either by weights fastened to the free end of the vibrator, or by varying the length of the vibrator at will.

A condenser (the capacity of which is governed by the frequency and strength of primary battery) is used with this type of interrupter.

Having decided upon the size of the core, and the amount of primary current at one's disposal, one enters into the calculation of the size of wire to be used in the primary. The gauges of wire usually run from No. 14 for large coils to No. 18 for small ones.

As the primary resistance is low, and the number of layers usually two, the most efficient winding for getting the best results out of the core, with a specified frequency of interruption, is easily decided by experiment. The magnetic influence of a primary core may be readily tested with iron fillings, or by suspended needles placed within reach of the lines of force.

Tests have proven that the most vigorous lines of force play near the ends of the primary core, while at the middle the magnetic curve drops close to the core. Hence we are led to distribute the wire of the secondary where the influence is the strongest.

And if the secondary is wound in two separate bobbins, each bobbin being wound to twice the primary diameter; - each bobbin being as long as its diameter and each bobbin placed one-half its length from the end of the primary core, the core being of such a length that the space between the bobbins will equal about one-half the diameter of the secondary bobbin, and with the insulation in all parts of the secondary perfected, as previously noted in this article, the result should be a coil suited to all the experimental needs of the amateur electrician.

It is, therefore, considerable of a question, in view of the facts above stated, whether the amateur of limited experience should attempt the making of a secondary coil, other than of very small size and capacity, and this is especially true if a secondary adapted to his needs can be purchased at little or no increase over the cost of material. As experience is gained, and a study of coil windings and their action becomes desirable, then constructive work with secondary coils can be taken up to good advantage.