Eberhart's Manual of High
Noble M. Eberhart, M.D., Ph.D., D.C.L., 1911
The Development of the High Frequency Current; Leyden Jars; Plate
Condensers; Electrical Oscillation. The Development of High Frequency Therapy.
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
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
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.