Simple Explanation of High Frequency Current

Within recent years some very astonishing things have been done with high frequency electric current. It is extensively used by medical practitioners, for wireless telegraphy, etc., and seems to act in a manner widely different from the ordinary electric current now more or less familiar to all. The following explanation of high frequency current will be found perfectly simple and will enable the average person to understand the seemingly magical things done, which are often seen at electrical shows and like exhibitions.

When the current flows in one direction in a conductor and then reverses and flows in the opposite direction it is said to be an alternating current. The number of times per second that the current makes a complete change of the direction and comes back to its original direction of flow is called its frequency. The frequency of commercial alternating currents is seldom over a hundred per second, and generally less. A high frequency current is one in which the frequency is numbered in the thousands or millions per second, although there is no strict dividing line where high frequency begins and low frequency leaves off.

Machines that produce high frequency currents directly are called high frequency alternators. Very few of them have ever been made, other methods having been devised for changing ordinary low frequency currents into those of high frequency.

The general method used is as follows: What is known as a step-up transformer is used to raise the voltage of ordinary low frequency current to a pressure of several thousand volts. This current is used to charge a condenser which consists of either the common Leyden jar or its equivalent in plates of glass, ebonite or mica coated with tinfoil.

When such a condenser is discharged through a suitable coil, high frequency current is produced. A Tesla coil for this purpose consists of a primary winding of a few turns of heavy wire and a secondary of finer wire of many more turns spaced apart to prevent short circuits between adjacent turns. Coils of this kind are often immersed in oil for insulation. There is no iron core of any kind in a Tesla coil. When the condensers are discharged through the primary of a Tesla coil in series with a spark gap, which is simply a break in the circuit over which a spark jumps, a current of enormous voltage and frequency is produced in the secondary. With this current can be produced many interesting and peculiar phenomena. Fig. 1 shows at the left the transformer for raising the pressure or voltage of the ordinary current to several times its original value. Next is the glass plate condenser, and at the extreme right is the Tesla high frequency coil.

When one terminal of the Tesla coil is grounded, long specter-like streams of sparks dart into space in all directions from the other terminal. If the capacity is increased these streams resolve themselves into flames making a striking display as shown in Fig. 2.

Nikola Tesla, while in Colorado carrying on a series of experiments, produced discharges 23 ft. in length, passing a current of 800 amperes at the tremendous pressure of 10,000,000 volts, producing a roar that could be heard for miles. At ordinary frequencies it is well known that current at a pressure of 1,000 volts and over will cause instant death. If, however, the frequency is very high, instead of producing death there is no sensation whatever and the current can be handled with impunity.

In the electrocuting of criminals a voltage of about 2,000 is used and only one-fourth ampere of current is sufficient to cause the death of any man. If an ordinary 16 c. p. lamp is held in the hand by one terminal and the other connected to only one terminal of the Tesla coil the lamp lights up brilliantly, consuming fully one-half an ampere or double the current used for electrocution, besides being painless. If any one doubts that the current passes through the body another method of holding the lamp will convince him. If the terminals of the lamp are connected to two persons and the terminal of the coil is connected to either one of them the lamp burns at full candle power. Even fine iron wire can be melted in two in the same way.

When the coil is in operation and is approached by a metal rod held in the hand the spark of the coil, which may be of over a foot long, will jump to the rod and entirely disappear if the terminal is touched, showing that all the current passed through the body.

With the pressure of this coil over 100,000 volts, if one person touches the terminal of the coil and another person comes near, sparks jump to the second person when several inches away. If a vacuum tube is held in the hand in the vicinity of the coil it will glow with a soft white light, but if the coil is touched the effect is greatly enhanced, so that it is possible to read by it. Geissler tubes are affected the same way.

This current would rather jump a considerable distance through air than go through even a few turns of heavy wire on account of the choking effect. This effect can be shown in a remarkable manner by connecting an ordinary 16 candlepower lamp in series with a piece of heavy wire bent in a circle. This lamp, although short circuited, can be lighted to full candlepower by touching it to the coil, showing that the current would rather go through the lamp of high resistance than through the heavy wire whose resistance is negligible.

When a Tesla coil of considerable power is in operation sparks can be drawn from metal objects in the vicinity, although many feet away.

Wireless telegraphy is a practical application of high frequency currents. In wireless telephony frequencies of from 100,000 to a million are used.

In electrotherapeutics the high frequency current is valuable.

Generally the source used to generate the high pressure necessary to charge the condensers is an X-ray coil, when used in electrotherapeutics.

It is held by many, Tesla among the number, that high frequency currents will work a revolution in the application of electric force, and that the electric power of the future will be produced in high frequency generating plants, will be sent out through space without transmission wires and will be collected at will at any point desired.