Oscillatory Currents and Some of Their Phenomena - II

In the circuits shown above the condensers have been considered as consisting of two or more sheets of metal separated a short distance from each other by insulating material. Any such condenser, even when the separate sheets have a small area, will have a capacity large enough to make the oscillations much slower than is desirable for many forms of experiments which may be performed with oscillatory currents. When therefore it is desired to obtain the most rapid oscillations, such as are used for the production of electric waves (about which more will be said later), it will be found convenient to arrange the circuit as indicated in Fig. 6.

Here S S are two sheets or spheres of metal; s is the discharge gap and l l is the wire along which the very rapidly oscillatory currents flow. The two metal sheets, together with the surrounding air, form a condenser in all respects like the condenser in E, Fig. 1, except that it has a very much smaller capacity on account of the great distance that the plates are separated from each other and from other conducting matter.

Another condenser, C, is indicated as placed in series with the secondary of the transformer T. The importance of this condenser rests on a practical point in the operation of the contrivance, as shown. The author has found that the secondary of a transformer generally gives such a large current that when the air gap s breaks down an arc of very low resistance is permanently established across this gap if the condenser C is left out, and that no true oscillations take place in the wire l l. By inserting a small condenser, as C, in the circuit of the secondary of the transformer the current becomes cut down to a low value, while the distance that the spark can jump in the spark gap s is not diminished. In this case the arc does not form permanently across s and good oscillations are obtained. A blower or polished balls or other devices often used to break the arc will then often not be required.

The customary way of exciting oscillations in a circuit like the above is to employ in place of a transformer an induction coil, as shown in Fig. 7. This is the arrangement first used by Hertz in his experiments on electrical waves. An induction coil will not, however, give as good or as powerful effects as a transformer. The chief reason for this is that the induction coil only supplies energy to the circuit in which the oscillations take place, at comparatively long intervals, the discharge from an induction coil not being continuous, as it practically is from a high-potential transformer.

Oscillatory currents may be obtained also from a Holtz or influence machine. Arranging the circuits as in Fig. 8, where b b are the terminals of the influence machine, very rapid oscillatory currents of small energy may be obtained in the circuit W W W W, but of sufficient power in the case of a large machine to excite vacuum tubes and X-rays. In the case shown the induced current will pass through the vacuum or X-ray tube when the spark passes between b b.

I shall now show some devices which ought to give oscillatory currents of great power, although I have never had an opportunity to actually test the matter. I offer the devices as of sufficient interest to be worth testing by experiment.

In Fig. 9 D is a direct-current dynamo of any voltage, I is a coil of a great many turns of heavy wire wound on an iron core, c c. C is a condenser, s a spark gap between balls about an eighth of an inch apart. P is a coil of a few turns of large wire and f a coil of fine wire. K is a device by means of which two carbons may be made to touch each other and then be drawn quickly apart until the arc which forms on separating them is broken. Now it is well known that when a direct current is passing through a coil wound on an iron core and of a great many turns, that on suddenly breaking the circuit the current which is flowing in the coil tends to keep on flowing in the same direction with such persistency that a spark of considerable length will occur at the point where the circuit is broken. Hence if the carbon points at K are brought into contact and a large current flows through I, on quickly separating them an arc of low resistance will first form between the points and, when the separation becomes great enough, will quite suddenly break. The current in I will, however, tend to keep on flowing and as it has no other path to take will flow into the condenser C, charging this to a high potential, and, if matters are rightly arranged, this potential may be made great enough to cause a discharge to take place across the air gap s. If this occurs the condenser will discharge through the coil P with an oscillatory current which will induce a current in the coil f. Thus a long discharge between the terminals a a may be obtained. It would be desirable to have the break in the circuit which is supposed to occur at K made at a number of different points at once. Devices for accomplishing this anyone can readily design.

In Fig. 10 A is an alternating current dynamo, T a commercial transformer which will give across the terminals of its secondary a potential of 100 volts. L is a very large coil of wire wound on an iron core, or what would serve the purpose as well, the secondaries of several transformers connected in series. C is a condenser of moderate capacity, P a coil of a few turns of heavy wire, f a coil of fine wire and s a spark gap. Suppose that a momentary current flows through the primary of the transformer T, then a momentary E. M. F. will act in the secondary of the transformer, tending to start a current flowing through the circuit, including L, C and P. The condenser C will be charged to a low potential and will then immediately discharge, causing a weak oscillatory current to circulate in the condenser circuit. Assume now that each time this oscillatory current starts to flow in one direction or the other a new current is sent through the primary of the transformer in such a direction that the E. M. F. induced in the secondary of the transformer helps the current which is already flowing in the secondary along in the same direction. This current will then evidently be increased. If we start a rocking chair rocking and each time it starts to go back in one direction give it a slight push it will be made by very slight impulses to rock back and forth a great way. If the impulses given to the chair are not rightly timed it will almost be brought to rest. Just so in the above electric circuit. If the oscillatory current has a natural rate of swing back and forth in the circuit and the alternating current through the primary of the transformer changes its direction at the same rate as the oscillatory current naturally swings, the surgings of this latter current will become very great and the condenser C will become charged to a very high potential and with a large quantity of electricity. The potential to which the condenser may be charged, by rightly varying the natural period of its circuit until it is the same as the period of the alternating current through the primary of the transformer, may be made fifty times as great as the potential which could be obtained from the secondary of the transformer alone.

Or, in other words, the potential between the points c and d may be made fifty times as great as the potential between the points a and b. If the potential between a and b is 100 volts the potential between c and d would be great enough to cause a discharge to take place across the air gap s, and this would cause a much more rapid series of oscillations to take place through the coil P. These could be used to induce a very high potential in the coil f. The circuit may be adjusted to resonance, as it is called, with the alternating E. M. F. in the transformer, by varying the number of turns in the coil L or the amount of iron in this coil, the capacity of the condenser, or, where it can be managed, by varying the speed of the alternator A.