Eberhart's Manual of High Frequency Currents
Noble M. Eberhart
Chapter One
Definition of High Frequency Current; Alternation;
Cycle; Period; Oscillation; Frequency; Lighting Vacuum
Tube Not Proof of High Frequency Current.
Fig. 1 - Large Tesla Coil
    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 currents.
   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 line.
   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.
Fig. 2 - Portable High Frequency Coil.

   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 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 type.
   At a frequency of about 10,000 cycles these tetanic contractions disappear and above that frequency there is neither pain nor gross muscular contractions.
Fig. 5 - High Frequency Coil, Giving All Modalities

   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 frequency currents.
   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 Chapter III.

Fig. 5a - Prismatic Ray Type of Portable Coil.