HIGH-FREQUENCY OSCILLATING CURRENT. This term is especially applicable to electrical currents, the high-frequency inter ruptions of which are obtained by means of con denser discharges in contradistinction to those produced by a disrupted static current, without the interposition of a metallic condenser in series with one or both terminals. The latter differs in several characteristics and is essen tially a high potential current, 10,000 to 50,000 volts, with a minimum amperage, usually about .0005. The usual alternating current in com mercial use in the United States is that known as 60-cycle: that is, it makes 60 complete journeys— there and back, so to speak —in each second. It has changed its direction, therefore, 120 times in the second. The high frequency current changes its direction 20,000 times or more per second.
To generate a high-frequency current it is usual to charge two Leyden jar condensers with a high potential current, the source of which may be a static machine or induction coil, shunting the two wires with a spark-gap for the purpose of disrupting the current. The external armatures of the condensers are short circuited through a solenoid or helix consisting of a few turns of very coarse copper wire (D'Arsonval). The helix may be substituted by a straight copper bar (Sheldon). The helix prolonged to from 50 to 100 turns constitutes the resonator of Oudin. The upper part of the helix resonates in unison with the lower, when properly in tune, as would a tuning-fork reso nate with another of the same pitch. The office of the extended helix is to amplify the current.
Another arrangement is after Tesla: The primary of a specially constructed induction coil is energized by an alternating current. The secondary terminals, giving a potential of 15,000 volts, are connected, one to each side of a suit able condenser. From each of these terminals a shunt is taken. One leads to one end of the primary of a Tesla coil; the other, broken by a spark-gap, is connected to the second terminal of the primary. The frequency obtained from the Tesla apparatus is fabulously high—mil lions a second. The potential may be hundreds of thousands or millions of volts. The amper age is sufficient to light to full candle-power several incandescent lamps. The primary of the induction coil consumes 15 to 25 amperes. For maximum high-frequency effects this type of apparatus seems to be essential. There are many modifications of this apparatus.
At the terminals of a working high-fre quency apparatus is seen a beautiful brush dis charge or effiuve of a peculiar violet color, which will leap to any object brought near it.
These brush discharges range from 20 inches upward to 50 inches in length. Interpose a plate glass one-half inch thick (or more), and the effluve will penetrate it. If the object be a vacuum tube it will glow almost As brightly as if nothing intervened. It is an ideal current for igniting Geissler and low-vacuum X-ray tubes; only one terminal need be connected.
One characteristic of the high-frequency oscillating current is its lack of power to excite the motor nerves, and, aside from a slight sen sation of warmth, the sensory nerves at the point where the electrode touches the body. The accepted explanation for this fact is, that the nerves respond to certain frequencies of stimulus; for the motor nerves, reaction takes place up to about 5,000 frequencies; if these are gradually increased muscular contraction di minishes and finally ceases. This theory is in line with reasoning as to the cause for action of the special senses — sight and hearing.
The high-frequency discharge is a rich ozone generator, and, applied to unhealthy granula tions and various skin diseases, acts as an oxi dizer, antiseptic and disinfectant. Applied to the skin before incision, it will render the site aseptic. In the hands of the expert electric phy sician the high-frequency current is used to remedy several diseased conditions. The vacuum-tube is employed to increase arterial tension, the D'Arsonval current to decrease it. The former is productive of a powerful tonic effect in cases where the blood pressure is nor mal, but is dangerous where arterio-sclerosis exists. The vacuum-tube application increases the blood supply to the part under treatment, resulting in an improvement in nutrition at that point. Its application is also marked by an in crease of the functions of excretion and secre tion. The high-frequency current has been used successfully in renewing the growth of hair where the follicles have not been destroyed, and in many cases has restored the original color to hair which had turned gray. The treatment is also efficacious in corpulency. For a general tonic effect the patient is placed upon an auto-condensation couch or in the centre of an auto-conduction cage. In the treatment of sub-acute and chronic rheumatism, sciatica, neurasthenia, etc., it is most useful. (See axe 1120THERAPEUTICS). Consult Curtis, T. S., 'High Frequency Apparatus: Its Construction and Practical Application' (New York 1916) ; Transtrom, H. L., 'Electricity at High Pres sures and Frequencies' (Chicago 1913).