Eberhart's Manual of High Frequency Currents
Noble M. Eberhart
1911
 
Chapter Two
 
The Development of the High Frequency Current; Leyden Jars; Plate Condensers; Electrical Oscillation.
The Development of High Frequency Therapy.

    The therapeutic value of the high frequency current depends upon a number of physical phenomena, some of which were known many years before the high frequency was thought of.
   In looking backward over electrical history, there are three points that bear particularly upon the development of high frequency therapy.
   First: The invention of the Leyden jar, or an electrical condenser; secondly, the discovery of what is known as electrical oscillation; and finally, its application to the human body.
   The Leyden Jar or condenser.  The Leyden jar was discovered in 1775 by Musschenbroek, and takes its name from the City of Leyden.  It consists of a glass jar covered on both the outside and the inside with tin foil. (Fig. 7).  These coverings only extend part way to the top of the jar.  A chain from the cover of the jar connects with the inner layer, terminating above in a small rod with a ball tip.  This is for the purpose of charging the jar by contact with the charging source or discharging it if this knob is brought nearly or quite in contact with a metallic conductor touching the outer layer.
   The peculiarity of the Leyden jar consists in the fact that when a charge of electricity is placed on one of its layers, another charge of opposite polarity immediately appears on the other layer of the jar.
   For instance, if the inner layer receives a positive charge, a negative one will be found on the outer lead foil and vice versa.
   These charges will be held for some time unless something occurs to connect the two layers and allow the opposite kinds of electricity to neutralize one another.
   In the Leyden jar, then, we have two charges of electricity separated from one another by the glass, which, although it keeps the charges from getting to one another, does not prevent their exercising an attraction upon each other; or, to speak more precisely, the one charge induces an opposite charge on the other layer.

Fig. 6 - High Frequency Coil, Giving All Modalitiesl.
 
Fig. 7 - Leyden Jars, Cylindrical Type.

   A substance which separates two charges of electricity in a condenser while still permitting them to have an influence on one another, is called a di-electric.  Other di-electrics than glass are mica, vulcanite, etc.  The contraction of the charge on the layer of the jar causes a crowding together or condensing of the electrical ions, and thereby gives rise to the name condenser for the Leyden jar or the other form known as the plate condenser.
   In the plate condenser we have two layers of tin foil separated by a plate of glass, thus keeping up the same relative arrangement that exists in the Leyden Jar.  To make the analogy more complete,

 
Fig. 8 - Section of Jar.
Fig 9 - Section of Plate Condenser. 
 
    I have been accustomed in my classes to make use of the illustration in Fig. 8, showing a section of a Leyden jar; and then supposing that this were made of flexible material, let us imagine that we took hold of the ends of the section and straightened it out when it would appear as in Fig. 9, which represents a cross section of a plate condenser.
   Electrical Oscillation.  When a Leyden jar or other condenser is discharged through an air space, there is apparently a single spark passing across the gap.  From the time of the discovery of the jar in 1775 until 1842, this was supposed to be the case.  At this time Professor Henry announced in reality there was a series of sparks passing back and forth between the terminals of the spark gap.  This phenomenon has received the name electrical oscillation.  It has been compared to the action of two columns of water of different heights connected at the bottom by a pipe with a valve in it.  When the valve is opened, since water seeks its own level, the higher column descends, and the lower column rises.  As a result of the action of gravity on the greater weight of the higher column instead of the column descending until level with the other column and then stopping, this force carries it below until it becomes the lower column and the other column the higher one, and thus the two columns balance back and forth until finally they come to rest at a level.
   Another illustration may be made use of in the pendulum, which when raised to one side and released, swings over to the opposite side nearly as far, and so back and forth through shorter and shorter arcs until it finally stops in the center.
   Electricity, following the same principle as it passes from a higher to a lower potential, produces a similar oscillation.
   Lodge gave an especially suitable illustration by likening the action to the vibration of a straight steel spring fastened at one end.
Fig. 10 - Large High Frequency Outfit
 
 
Fig. 10a - Portable Coil.
 
   Electrical oscillation is the keynote in the therapeutic application of high frequency currents, although it was nearly forty years after its discovery before anyone thought of applying the principle in the treatment of the human body.