The D'Arsonval current may be obtained from an induction coil, static machine or trans former, capable of producing sparks four to eight or more inches long. The two poles are connected with the inner coatings of two Ley den jars whose outer coatings are connected with the extremities of a solenoid or cylindrical spiral of perhaps 20 turns of coarse wire about IA inch apart. Conducting cords lead to the patient from each of the extremities of the solenoid or from any two turns of it. The apparatus but not its connection with the patient is similar to that shown in Fig. 4.
The inner coatings of the two Leyden jars are oppositely charged and are connected with the terminals of a spark gap of adjustable length or preferably, an adjustable number of short spark gaps in series. With each impulse from the induction coil, the Leyden jars are charged to such an extent that a discharge of their inner coats occurs through the .spark gap. This releases the opposite charges in the outer coatings of the two Leyden jars and their high frequency discharge passes partly through the solenoid and partly through the parallel path formed by the patient's body.
One would suppose that with each dis charge of the Leyden jars the current would almost all pass through the solenoid which is a relatively short length of thin wire whose resistance measured in ohms is trifling com pared with that of the human body. There is however, a counter-electromotive force gene rated in a coil of wire at the malting of the current which adds a great inductive resistance to the passage of the current and increases enormously the percentage of the current which will pass through any other path open to it. The current passing through the patient may be measured by a hot wire milliamperemeter and may be all the way from 50 to 1,000 or more milliamperes. It is also a current of much higher voltage than the galvanic and the fact that it does not cause muscular contraction or a shock of any lend is due to its very rapid oscillations, a million or more per second. The motor nerves respond actively to alterna tions of 20 to 2,000 or 3,000 per second but are inert in the presence of much higher than 5,000 oscillations per second. The oscillatory character of the current prevents the elec trolysis which would render a constant cur rent of 1,000 milliamperes intolerable. One principal effect of the passage of the D'Ar sonval current through the tissues is the pro duction of heat in the tissues themselves by ohmic resistance of the same nature as the production of heat by a current passing through a galvano-cautery wire or through the filament of an electric light bulb. Whether all the
other effects are due to this generation of heat or whether some are due to direct stimulation of the tissue cells, is uncertain but the effects themselves are of the greatest value in therapeutics. D'Arsonval high frequency cur rents of moderate milliampereage may be ap plied by two wet electrodes or by bare metal electrodes; or by one glass electrode and a glass vacuum bulb or by autocondensation. Diathermy (or less properly called thermopene nation) is simply a D'Arsonval high frequency current of very great milliamnereage tending consequently to produce a great deal of heat in the tissues.. Two metal electrodes may be ap plied to a piece of raw meat and after the cur rent has passed through for some time, the meat will be found to be cooked all the way through. The electrodes themselves do not become too wann to be touched by the finger — are not warm at all until contact is made with the meat. This is the underlying principle of the Deforest Cold Cautery which is suited to all the purposes of a hot metal cautery.
Diathermy may be applied by two wet elec trodes, for instance at opposite sides of a joint, for producing a local effect. Or it may be used for producing a constitutional effect by means of Autocondensation. One conducting wire passes to a very large metal surface close to the patient's body but separated from it by a thick sheet of some complete and impenetrable nonconductor The patient may hold one or more metal terminals from the other pole of the D'Arsonval apparatus or may be in close proximity to a second insulated surface charged from the other pole. During the application currents of high frequency, medium tension and higil milliampereage (about 1,000) surge through the patient's body. The application produces very little sensation except a slight warmth and no shock or muscular contraction. The metal terminal must, however, be grasped before the current is turned on and not re leased during the flow of the current. Other wise the patient will receive a succession of ex ceedingly hot sparks.