H. E. DILL

The Ruhmkorff coil, with its many turns of fine wire coiled in sections and combined together to form a secondary, has been so long an indispensable part of a well equipped electrical laboratory that I shall make no attempt to describe in detail all of the principles of operation, but refer the beginner to one of the numerous elementary works upon electricity. It is advisable, however, to point out the basic laws as discovered by Faraday and others seventy years ago, in order to make clear the general arrangement of all coil windings.

First of these principles is where you have two distinct circuits contiguous to each other, but not in contact, and by exciting an electric current in one of them instantly induce, that is, produce by induction, an electric current in the opposite direction in the other. And if the current in the originating circuit is suddenly interrupted, a secondary current will momentarily be induced in the secondary circuit, but in this case in the same direction as the first current. It follows that if we alternately open and close the primary with some rapidity we shall induce in the secondary circuit a current continually changing in direction. As to the characteristics of these alterna-nations further observations will be made later.

The second principle relates to the rapid movement of a magnet in proximity to a conductor or of a conductor in proximity to a magnet - developing an electric current in the conductor.

Thirdly, a bar of iron may be rendered powerfully magnetic by being placed in the vicinity of an electric eurrent. This magnetism will continue while the iron remains under the inflnnence of the current.

In a device possessing these features, namely, a soft iron core; a coil of wire encompassing the core; a source of current to pass through the coil, thus energizing the core; a secondary coil of wire insulated from but encircling the primary winding and the core; a means of interrupting at will the source of the primary current, - these are the factors in induction coil building.

The actual E. M. F. of the primary circuit remains practically unaltered, but the E. M. F. of the resulting secondary current is greatly increased from several causes. One is the great length of wire in the secondary, coiled in many turns in the field of magnetic influence. Second, the rapidity of interruption constantly produces a magnet in proximity to the secondary winding and as quickly takes it away, and this rapid magnetization and demagnetization of the core intensifies the current already intensified by the cutting of lines of force. It is to be remembered that this continual making and breaking of the primary circuit causes the secondary to be further intensified by the alternations of current.

In coils bearing condensers in connection with core-energized interrupters, the condenser plays an important part in intensifying the secondary circuit, for as previously explained, the induced alternate currents depend on the make and break of the interrupter, but it is also a fact that the breaking of the primary circuit momentarily produces by induction a slight extra current in the same direction, in itself a phenomenon called self induction. This self-induced current is absorbed by the condenser at the moment the primary circuit is broken, only to be discharged a moment later through the primary coil, thus creating a current in the opposite direction to the battery current, and thus assisting in demagnetizing the core and greatly adding to the efficiency of the coil.

Having now explained the method of intensifying a source of current by induction, I will describe one type of interrupter, whose invention some years ago opened a new era in high fre-quency work. I refer to the Wehnelt or Electrolytic break, for which is claimed interruptions as high as 2000 per second when operated direct connected with incandescent mains.

As far as personal experience points out, when used with ordinary coils this interrupter varies from 250 to 1500 per second, depending upon the size of the electrodes (to be described), the induc-tence of the circuit and the value of the E. M. F, With specially designed coils, frequencies far in excess of these have been reached in the secondary discharge by using special capacities in that circuit. The subject of calculating these frequencies is now a foremost problem in this branch of science. The pitch of an interrupter is a rough range or measure of its breaks, and it is reasonable to suppose that its range of sounds is the limit of its rate of break, but where the variance of sound precludes estimate, by what are we to be governed? It is not advisable at present to describe the complex devices for frequency measurement, because many of the resnlts have been questioned, although the rotating mirror method and calculations based on the inductance and capacity values have demonstrated that sinusoidal currents with a frequency of 400,000 cycles may be produced by special and elaborate apparatus.

In the older forms of vibrators using platinum contacts, primary voltages of 100, or more, gave but meagre results owing to the heating and blackening of the platinum and the low rate of interruption, but with the introduction of the Wehnelt there was opened a new field of research.

This interrupter, a sketch of which is here given, consists of a glass jar, filled with water slightly acidulated with sulphuric acid. In this solution is immersed a lead plate and an insulated platinum point, the plate being connected to one terminal of the electric current supply and the platinum point to the other. A suitable adjustment of the two electrodes gives rise to current interruptions of a high and varying frequency.

Because of the effect of current action on the platinum tip, and being unable to find a less expensive and equally satisfactory metal, many experimenters were led to substitute in its place a small glass bottle having an extremely small hole drilled near its base. In this bottle was placed a strip of lead which was connected with one side of the source of current.

The conductivity of the electrolyte was not only benefited by the addition of more sulphuric acid and a small quantity of magnesia sulphate to the water, but the number of interruptions seemed also to be increased and operations started at a lower voltage.

As to its general adaptation to amateur work, it is well to note that it is generally the custom in designing a high potential coil to consider frequency of interruption and insulation together for the penetrative effect of a rapidly broken magnetic field may imperil an insulating medium ordinarily secure under a low rate of interruption. Oil used as a dielectric, saturates the windings and being liquid becomes self-sealing should by chance a stray discharge pass from one part of the secondary to another. Hence the use of transel oil and similar oils in many grades of transformer work.