John E. Atkins

The amateur interested in electricity should make himself familiar with the various parts of a spark coil; it is proposed, therefore, while avoiding as far as possible electrical technicalities, to explain understanding-ly the principal features and functions of the coil.

The first step is to acquire a supply of electrical en-cnergy to produce the required current. Assume that one of the many forms of primary battery is selected - preferably new, dry cells. The next question is how the electrical energy, with which the amateur has supplied himself, is to be made to produce a spark by means of an induction coil.

The construction of the induction coil is based upon certain principles of electricity discovered years ago.

The first principle is: - With two entirely separate and distinct circuits placed near to each other, but not in contact, by exciting an electric current in one of them, there will be instantly induced, or in other words, produced by induction, an electrical current in the opposite direction in the other circuit. The original current is called the primary current, and the induced current is called the secondary current, and the circuit in which it is induced is called the secondary circuit. Similarly, if the current in the primary circuit is suddenly interrupted, a secondary current will be momentarily induced in the secondary circuit, but this time in the same direction as the primary current. It follows that, if you alternately open and close the primary circuit with great rapidity, thus alternately exciting and interrupting its current, there will be induced in the secondary circuit a current which is continually changing in direction ; in other words, what is called an alternating current.

The second principle is: - That the rapid movement of a magnet in proximity to a conductor, or of a conductor in proximity to a magnet, will excite an electrical current in such conductor.

The third principle is: - That an ordinary bar of soft iron, which is for practical purposes non-magnetic, may be turned into a very powerful magnet - termed an electromagnet-by being placed in the neighborhood of an electric current, and that the magnetism so produced will last so long as the current continues.

Now, in the center of the Ruhmkorff coil is a core of soft iron wire around which the primary wire is wound, the effect of which is that directly an electric current is excited in this circuit the iron core becomes magnetized. The core is, in fact, instantly converted into an electro-magnet emitting lines of magnetic influence, and the immediate sphere through which these lines of magnetic influence pass is called the magnetic field. The secondary coil, consisting of many thousands of yards of very fine insulated copper wire, is wound round the core and primary circuit in such a manner that it is continually passing through the magnetic field.

Fig. 1, while by no means showing the detailed construction, will serve to illustrate the principle upon which the primary and secondary circuits are respectively wound. The iron core is represented by a a. The thick line, marked b b, represents the primary circuit, the positive, +, and negative, -, terminals of which A A would be connected to the corresponding terminals of the battery. It will be seen that this primary circuit, b b, is wound round the core, a a, so that immediately the current is turned on the core will be magnetized.

Fig. 1.

Owing to the proximity of this secondary circuit e e to the primary circuit b b the instant the primary current is excited in b b, a secondary current in the opposite direction is induced in c c, while the instant the primary circuit b b is broken or opened, a secondary current in the same direction is induced in the secondary circuit, c c.

In order to make the diagram plain, the secondary circuit is shown as passing only twelve times round the primary circuit and core, but in actual practice this secondary circuit is several miles in length, and is wound many thousands of time round both, in the form of a bobbin.

The horizontal lines in Fig. 1 represent the invisible lines of magnetic force caused by the magnetization of the core a a , and it will be noticed that by the ar-arrangement above described, the secondary circuit, c c, is enveloped in this magnetic field; in other words, the secondary current is continually passing through the lines of magnetic force.

The foregoing is a mere outline of the principle of the construction of an induction coil, and the beginner will naturally ask how this apparatus operates to intensify the force of the current supplied by the batteries.

The actual E. M. F. of the primary circuit remains in all probability unaltered, but the E. M, F. of the resulting secondary current is vastly increased from several causes. One cause is the great length of the secondary circuit, necessitating an enormous number of turns or coils. The greater the number of coils, the more frequently does the secondary current have to pass through the lines of magnetic influence, and the more often it passes through this magnetic field the more intense becomes the voltage.

Another cause is a contrivance by which the primary circuit is opened and closed, and the primary current consequently interrupted with great frequency and rapidity, thus constantly magnetizing and demagnetizing the core. We have already explained that the rapid movement of a magnet in proximity to a conductor will excite a current in such conductor. In the Ruhm-korff coil you are, by repeatedly interrupting ;he primary circuit, constantly magnetizing and demagnetizing the core; in other words, constantly producing a magnet in proximity to the secondary circuit, and immediately taking it away, an operation practically equivalent to the rapid movement of a magnet in proximity to the secondary circuit, the result being to excite a current in the secondary circuit.

Fig. 2.

It will thus be seen that the secondary current is not only intensified by its constant passage through the lines of magnetic force, but also by the rapid magnetization and demagnetization of the iron core. It must also be remembered that the continual making and breaking of the primary current causes the secondary current to be constantly changing its direction, so that it is further intensified by being converted into an alternating current.

The device by which the primary current is thus interrupted is shown at Fig. 2. In Fig. 2, the primary circuit is closed. The current passes from the positive terminal, +, up the metal pillar, G, by way of the platinum points, H, to the hammer or contact breaker, J, down the spring of the hammer, D, and thence by way of the primary circuit b b, round the core a a, and back to the negative terminal, -.

A loop, d d, is shown which goes off to the condenser, e e, but if this is followed it will be perceived that the two parts of the condenser are not in contact, so that the current cannot circulate through this loop, the functions of which are described later.

Now, the moment the current is turned on, the core, a a, is magnetized, and, as a natural consequence, the hammer or contact-breaker, J, is drawn by magnetic attraction towards it. The immediate effect of this is to break the contact, or open the circuit at the platinum points, H. The result of thus breaking or opening the primary circuit is to demagnetize the core, a a, so that the hammer, J, ceases to be attracted, and is caused by the spring, D, to fly back to its original position, when the process is automatically repeated; the primary current being thus interrupted and the circuit opened and closed with wonderful rapidity.

It now remains to draw attention to the loop, d d, leading to the condenser. The reader will observe that this loop is not strictly a portion of the primary circuit proper, but a kind of extension of the circuit from the contact breaker. The condenser itself, e e, to which this loop leads, consists of several thin layer's of tinfoil placed in the base or stand under the coil. Each layer is connected to the next layer but one, but is carefully insulated from its next-door neighbor. One set of connected layers is attached to the positive conductor of the loop, while the other set is attached to the negative. It will be seen from Fig. 2 that by this arrangement this loop does not form a complete circuit, there being no actual contact between the positive and negative layers; whereas, if they were connected, the primary circuit would not be opened or broken when the interruptions at H, which we have already described, take place, because the current could pass from G to X) by way of d d and e e.

We have already explained that the making and breaking of the primary circuit induces alternate currents in the secondary circuit, and it is also a fact that the breaking of the primary circuit will momentarily produce by induction a current in the same direction in itself- a phenomenon which is called self-induction.

Now, the function of the condenser is to absorb this self-induced current which is formed in the primary circuit at the moment when such circuit is broken at H. When this break occurs, the current seeking a passage rushes to the condenser by way of the loop, d d, only to find that there is no contact between the positive and negative sheets of tinfoil which compose the condenser. The electricity thus accumulated by the condenser is discharged a moment later through the primary coil, thus creating a current in the opposite direction to the battery current, and consequently demagnetizing the core.

As the induced current is due to the change in the magnetization of the core, this demagnetizing current greatly adds to the efficiency of the coil.

Further, by thus absorbing the current, the condenser reduces the liability of the current to "arc" or spark across the space between the platinum points at H when the contact is broken.

The important part which this condenser plays in intensifying the secondary current will be appreciated when we inform the reader that a 1 in. spark coil, without a condenser, will barely give a spark of § of an inch in length.

To such an extent is the intensity of the secondary current increased by the devices which we have described, that if the secondary circuit is left open between the terminals F F, the current will be forced across the intervening space, and a continuous stream of sparks, like miniature forked lightning, will pass from one terminal to the other.