Eberhart's Manual of High Frequency Currents
Noble M. Eberhart
1911
 
Chapter Three
 
Types of Apparatus; D’Arsonval Type; Tesla Transformers; Impedance; Direct D’Arsonval Currents; Auto-Conduction; Auto-Condensation; Resonator of Oudin; Tuning Coils; Measuring the Current.
 
 
   Types of Apparatus.  There is no intention on the part of the author to encumber this volume with lengthy descriptions of apparatus.  For such information, the reader is referred to the manufacturers of electrical outfits.
   All high frequency instruments, however, are constructed along two principal types -- that of D’Arsonval and that of Tesla -- and these should be understood by the high frequency operator, since all machines represent one or the other or a combination of these types.
   In connection or combination with them, a third instrument, the resonator of Oudin, is in such common use that it also calls for special description.  In speaking of high frequency currents, to be specific, we should always state the form of current used, as D’Arsonval high frequency, or Tesla high frequency, etc.
 
 
Fig. 11 and 12 - Portable Coils.

   The D’Arsonval type of apparatus as it is manufactured today consists of a combination of the D’Arsonval solenoid and an improved type of the Oudin resonator.  The Oudin current derived from this type of apparatus serves to energize vacuum electrodes, the spray electrode, the so-called “static spark” electrode, the portable ozone inhaler, and also is the current which is utilized for obtaining the “cold” fulguration spark.  The D’Arsonval solenoid in this instance delivers a comparatively low potential and high milliamperage current.
   The Tesla type of apparatus is designed along the same general lines as is the D’Arsonval and has the same wide range of usefulness, but the general characteristics of the current are different in that the Tesla type of apparatus delivers a high potential, low milliamperage current.
   If every physician employing high frequency apparatus was aware of this difference it is the author’s opinion that many cases of misunderstanding and dissatisfaction would be cleared away.
   In other words, the milliamperage reading of the meter is not an indicator of the capacity or power of the apparatus.  This means that 500 milliamperes of Tesla high frequency current is just as capable of producing deep constitutional effects as 1,500 milliamperes on a D’Arsonval high frequency apparatus.
   The relative merit of the “Tesla” versus the “D’Arsonval” apparatus more particularly as regards auto-condensation and diathermy has been the subject of numerous discussions among electro-therapeutists.  The thickness of the di-electric (auto-condensation cushion) has also entered into this discussion.  It has been proved that either type is capable of producing results and in consequence thereof the author does not feel capable of choosing himself as the sole arbitrator in deciding which is better.  He however, for the past three years has been employing the Tesla type of apparatus in his individual practice and has been deriving eminent satisfaction from its use.  He employs a cushion of approximately three inches in thickness– this size being endorsed by the standardization committee of the American Electro-Therapeutic Association.
   As will be stated in Chapter VI, the author originated the thin cushion as well as the portable thick pad, and the method of making these different forms is given.  He bows to the ruling of the committee referred to above and now employs only the thicker cushion.
   The D’Arsonval Current is one of high frequency, not very high voltage, and high amperage.
   It is administered as a bi-polar method, thereby producing constitutional effects.  D’Arsonval started with an induction coil attached to the direct current.  The terminals of the secondary coil were attached to the inner layers of two Leyden jars, thereby charging one of them positively and the other negatively.  In the circuit between these two layers was placed an adjustable spark gap.  Between the outer layers of the jars was placed a solenoid or coil consisting of fifteen to twenty turns of coarse copper wire.  This combination of condensers connecting on one side through a spark gap, and on the other through a coil of wire, is an absolute necessity in order to obtain the D’Arsonval current, and it therefore constitutes this type.  See Fig. 13.
   When the positive layer of the first receives a sufficiently high charge, it jumps across the spark gap to neutralize the negative charge in the second jar.  Immediately the positive charge on the outer layer of the second jar is released and passes through the coil of wire to neutralize the negative charge on the outer layer of the first jar, and as it passes through this coil or solenoid, it produces a high frequency current, keeping in mind the fact that the oscillation back and forth through the condensers exists as described in Chapter II.  The patient is connected on that side of the condensers that discharges through the coil and is really on a shunt or switch from that part of the circuit, and the reason the patient receives any current at all, is because the solenoid possesses the property known as self-inductance, which impedes the passage of the current, allowing part of it to go through the lesser resistance in the patient’s circuit.  The process of interfering with the current as a result of the self-inductance is called impedance.

Fig. 13 - Diagram of D’Arsonval Type.

   The Tesla Transformer and Coil.  Tesla started with the alternating current, and by referring to our definition of high frequency currents (Chapter I), it is seen that all that was required was the increasing of the frequency to a point which we properly call high.  To do this, he used as a primary , a coarse coil of wire consisting of a few turns only, while the secondary coil consisted of an extraordinary number of turns of very fine wire–the result being that the current induced therein was of very high tension and very high frequency.  The Tesla transformer or coil is especially adapted to the alternating current.  It is employed in all of the portable high frequency apparatus on the market.  The Tesla primary resembles the D’Arsonval current except the amperage is less.  Fig 14 shows one method of evolving the various currents.
   Administration of the D’Arsonval Current.  There are three principal ways in which this current is applied to the body; first, the direct bi-polar method, which has also been subdivided into two or three forms; second, auto-conduction, and third, auto-condensation.  In the first method, the patient is directly connected to the terminals of the apparatus.  Whether the patient holds the two electrodes, or whether he is attached on one pole, while the other pole is connected with an electrode in the hands of the operator, the method is still called direct application.  It should be borne in mind that the direct D’Arsonval current is also known under several other terms, such as diathermy, electro-coagulation and thermo-penetration.  See Chapter XI.  Referring back for a moment to the component elements of the D’Arsonval apparatus. It is seen that solenoids and condensers are fundamental parts.  In the elementary D’Arsonval type we have one set of condensers and one solenoid.  It would appear, therefore, that the inventor said to himself; “If one solenoid gives a current of high frequency, let us add another solenoid and see what that will do.”  Thus the second type, auto-conduction, is constructed, and a large solenoid is connected in the shunt which forms the patient’s circuit, and it is made so large that the patient may be placed within this coil or cage when it is found that as the high frequency currents traverse the coil, other high frequency currents are induced in the body of the patient.  This is auto-conduction.

Fig. 14 - Diagram of Tesla Type

   Auto-condensation.  Again reverting to our elementary type instead of using the second solenoid, the next arrangement was the use of an extra condenser, and the current passes from one terminal into one plate of the condenser, while the other plate was formed by the body of the patient, hence the term auto-condensation.
   Oudin’s Resonator.  Oudin discovered that with a coil of wire properly tuned or adjusted to the coarse coil or solenoid of D’Arsonval, the electrical currents of the latter produced currents of such strength that they might be taken from the terminal of the larger solenoid and applied to the body.  They are of high frequency, high voltage and low amperage, resembling the Tesla secondary.  As ordinarily constructed, the D’Arsonval apparatus and the resonator are combined in one instrument; the resonator proper consisting of a large coil of fine wire placed above the coarse solenoid of the D’Arsonval machine.  See Fig. 15.
   Tuning coils.  The best results in resonance are obtained when the coils are properly adjusted and attuned to one another.  To facilitate this, the wire that passes from one side of the solenoid is attached to a sliding metal finger, which may be moved up or down on the solenoid, and thereby increase or decrease the number of turns of wire employed.  This permits proper adjustment of the coils, at the same time it is found that the greater the number of turns included, the sharper the resulting spark, and vice versa.

Fig. 15 - Diagram of Oudin Resonator.

   Measuring High Frequency Currents.  High frequency currents cannot be measured with an ordinary meter.  For D’Arsonval currents the method customarily employed is the use of a hot wire meter.  This is really a thermostat instead of an electrical device.  It is based upon the fact that the passage of the high frequency current gives rise to heat and this heat causes expansion in a platinum wire and thereby deflects the needle in proportion.  Hot wire meters are faulty, but give a relative idea of dosage when employed on the same apparatus.  One is shown in Fig. 16.  Other methods of measuring these currents are by Gaiffe and Meylon’s induction amperemeter, which “measures the repulsive force between the original current and the current which it induces in a coil attached to the indicator.”

Fig. 16 - Hot Wire Meter.
 
Fig. 16a - Air Cooled and Air Cleaned Spark Gap.
 
Fig. 17 - Large Combined Tesla, Oudin and D’Arsonval Type Coil.