Eberhart's Manual of High Frequency Currents
Noble M. Eberhart, M.D., Ph.D., D.C.L., 1911
Fig. 1 - Large Tesla Coil
Definition of High Frequency Current; Alternation; Cycle; Period;
Oscillation; Frequency; Lighting Vacuum Tube Not Proof of High Frequency Current.
What is High Frequency Current? A high frequency
current is an alternating (oscillating) current in which the frequency is beyond the point
of producing muscular contraction.
An Alteration is a complete reversal of the direction
of a current as any one of the waves shown in Fig. 3.
A Cycle is two alternations. It constitutes the round
or circle of the current, embracing one positive and one negative wave or alternation.
Oscillation. An oscillation is a series of diminishing
waves which flow alternately in opposite directions, but not necessarily in the same time.
Oscillatory currents periodically reproduce similar series of waves as shown in Fig. 3.
Their frequency is very high and is the reciprocal of period. It is computed on the number
of double alternations, thus corresponding to the method employed with low frequency
Period is the time required for one cycle.
Frequency is the number of complete cycles occurring
in one second of time. It is the reciprocal of Period.
Explanation. In order to understand the foregoing
definitions it will be necessary to recall some fundamental facts.
The direct current is one which always acts in one
direction, that is from positive to negative, and is graphically represented by a straight
The alternating current, in contrast to this, is one
in which the current is constantly changing its direction or polarity, and we represent it
by a waved line (Fig. 3), the waves above the line being positive waves or alternations
and those below the line negative waves.
Let us call the line zero in volts and the crest of the wave 110 volts.
The current starts at zero, reaches the maximum positive voltage and then reverses and
goes back to zero and repeats on the
Fig. 2 - Portable High Frequency Coil.
negative side. This complete reversal of the current is called an
alternation and two of them make a cycle. If we only represented two alternations as in
Fig. 4 and then made the second one return to the starting point as shown by the dotted
line we would have in this case a circle, and the word circle will keep the meaning of
cycle fixed in the mind. Alternations do not necessarily assume the shape of a semi-circle
but may represent any form of wave, still two of them represent the completion of the
circuit or cycle. In high frequency currents these cycles become a succession of
oscillations or undulation as represented graphically in Fig. 3.
Frequency is a term properly belonging only to the
alternating current. We have uni-directional currents such as those derived from induction
coils, in which the current is broken up into a rapid series of waves, with those which
would be below the line damped out, leaving a succession of pulsations. This current
lights up vacuum tubes and performs much of the same work as the high frequency, but is
not a true high frequency current.
These are called pulsatory currents and in
place of the word frequency we indicate their rapidity by the term periodicity.
To return to our explanation of the alternating current let
us suppose that it takes one-one hundred and twentieth of a second for any one of the
alternations in Fig. 3. Then two alternations or one cycle would take two times one-one
hundred and twentieth, which equals one-sixtieth of a second, and this is the period of
the current illustrated. If it takes one-sixtieth of a second for one cycle, there will be
sixty times as many in one second, and this is the frequency.
Fig. 3 - Alternating and Oscillating Currents Fig. 4 - Analogy Between Cycle and Circle.
It will be seen that if we have the period of the current, all that is
necessary is to invert the fraction and we have the frequency or number of cycles.
In the illustration we have arbitrarily represented an
alternating current of 110 volts, sixty cycles, which is the ordinary commercial
alternating current employed in electric lighting. It is a low frequency current.
Other low frequency currents have 25 cycles, 125 cycles 133 cycles, etc.
Low, Medium and High Frequency Currents. If we apply a
low frequency current to a muscle we find that the muscle contracts, and this is powerful
and may be strong enough to be painful. As we increase the frequency of the current the
painfulness decreases, but more than a single muscle tends to contract. Those are currents
of medium frequency of which the sinusoidal current is a
At a frequency of about 10,000 cycles these tetanic
contractions disappear and above that frequency there is neither pain nor gross muscular
Fig. 5 - High Frequency Coil, Giving All
The absence of pain is supposed to be due to the inability of
the sensory nerves to comprehend such rapid alternations just as we have vibrations that
cannot be recognized by the auditory nerve as sound or by the eye as light.
In these higher frequencies the contractile effect is
expended upon the individual cells making up the tissues instead of on individual muscles.
This I call cellular massage and it is one important reason why high
frequency currents produce such a marked benefit on nutrition and metabolism.
As the current increases in frequency and voltage, other
peculiarities appear and it no longer requires a complete metallic circuit but is capable
of traversing long distances as ether waves as in the wireless telegraph. In fact, high
frequency currents are better understood as vibrations than as ordinary currents.
From the fact that muscular contractions cease with 10,000
cycles or oscillations, this has been taken as the dividing line between medium and high
As the frequency with some apparatus is estimated in
millions, I ave been advocating a higher dividing line, say 100,000, as more in
proportion, but this would cause considerable confusion at present and so the original
nomenclature is adhered to.
Lighting of Vacuum tube Not Proof of High Frequency
Current. As intimated above, exciting a vacuum tube as from the
static machine or from one terminal of an induction coil is not proof of the presence of a
high frequency current. To produce the high frequency current there must be attached to
either of the above one of the several forms of high frequency apparatus described in
Fig. 5a - Prismatic Ray Type of Portable Coil.