Eberhart's Manual of High Frequency
Noble M. Eberhart
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.
Fig. 6 - High Frequency Coil, Giving All Modalitiesl.
Fig. 7 - Leyden Jars, Cylindrical Type.
In looking backward over electrical history, there are
three points that bear particularly upon the development of high frequency
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
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.
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
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
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.