TCBA founder, Harry Goldman and the TCBA logo

TCBA - Tesla Coil Builders Association

Devoted to the construction, operation and theoretical analysis of the Tesla coil

TCBA Volume 10 - Issue 1

Page 13 of 18

Editorial Errors and Miscalculations

When Tesla coils were in their hey day, editors and engineering consultants to scientific publications often demonstrated a lack of understanding on this subject. Take, for example, the misunderstanding that existed regarding Tesla and Oudin coils. The following are two more examples of editorial errors and miscalculations. One has to do with Tesla's Colorado high voltage experiment. Tesla claimed 18,000,000 volts at 1100 (RF) amperes. In a letter to Science & Invention (December, 1920), a reader disputes the editor's claim of billions of volts and sparks 3' in diameter (whew!). The second example comes from the British Journal Model Engineer & Practical Electrician (March 31, 1932). A reader wants to know why the frequency of a Tesla coil is many times that of the power supply. He also requests the formula for determining the frequency. The editor failed on both counts. By 1932, the answers were readily available in numerous scientific papers and textbooks.

Magnitude of Tesla High Frequency Currents

(1072) James T. Wilson, Jr., Brooklyn, N. Y., writes this journal:

Q. 1. About an apparent error in the “Wireless Course,” by Gernsback, Lescarboura and Secor.

A. 1. With reference to the error which you cite as appearing in the Electro Importing Co.'s “Wireless Course.”

In the first place, the “Wireless Course” is being rewritten in considerable detail so as to include some valuable and timely material on vacuum tubes.

We note what you have to say concerning the statement on page 135 of the “Wireless Course,” concerning the giant Tesla transformer, and the coils which Dr. Tesla used in his “famous Colorado experiments, and in which you doubt that the discharge was of the magnitude of 800 amperes, at a potential of a billion volts or more.

We cannot vouch for the exact figures of this discharge, but you can figure it out for yourself as to the approximate potential and the amperage, when we tell you that Dr. Tesla's records and photographs which we have had the pleasure of seeing, show he absolutely produced tremendous discharges, the like of which have never been duplicated. They measure 100 feet in length in some instances, and the various sparks have diameters ranging from a few inches up to a foot or more.

Where most of the people who try to figure out the Tesla experiments mathematically err lies in not allowing for the great difference between the actual and the apparent watts, or in other words, they forget the low power factor, which the editor has often discust with Dr. Tesla; and further, the fact that these tremendous energy rates are dependent upon or based upon a very short time interval.

Simply explained, the theory of the apparatus and the tremendous discharges obtained, is as follows:

If you have 100 kilowatts in the form of low frequency A. C., and you use this energy for a period of one hour, then you would call the amount of energy expended 100 kilowatt-hours; but if you were able to charge this energy into huge condensers and then discharge it in less than a minute, or 1/60th of an hour, then the rate of discharge would be 60 times 100 kilowatts, or 6,000 kilowatts. Another thing in regard to the amperes of current passing thru a high frequency circuit, such as a Tesla circuit, is that the current does not act in the same way as a low frequency, 60-cycle A. C., or direct current, and is not computed in any such manner.

To prove this point in a simple way, take, for instance, the fact that if you apply sufficient potential to the human body, less than 1/10 ampere passing thru the heart causes death invariably. But Tesla, and thousands of others, have readily past tremendous currents thru and over the body with a current registering many more amperes than anyone would ever believe possible - in fact, to such an extent that whole banks of incandescent lamps have been lighted up by the current after it had past thru the body.

In other words, we enter into an entirely new realm of electrical calculation when figuring on Tesla currents and circuits.

4658. Tesla Coils. - N. K. (Whitley Bay).

Q. - (1) Why is the frequency of a Tesla coil many times that of a spark coil? Is it due to the extra current at the break of the initial primary circuit? (2) Is there any formulæ for working out the frequency? (3) Would two or more induction coils and condensers connected secondary of No. 1 to primary of No. 2, and so on, with, say, a 1-1 ratio, increase the frequency, as I take it the voltage or ratio of windings has nothing to do with it?

A. - (1) The high frequency of current in a Tesla circuit is due to the effect of a condenser, such as a Leyden jar or jars, connected with a spark gap in the secondary circuit of the induction coil. The spark discharge from a condenser is oscillatory, the frequency of oscillation is extremely rapid, in the region of a million oscillations per second. The voltage can be raised by means of a Tesla transformer, consisting of a few turns of thick copper wire constituting the primary, placed inside a coil of many turns of fine wire constituting the secondary. You will find a description with drawings of a small high frequency Tesla outfit, with diagram of connections, in the Model Engineer of October 7th, 1915. The price of a copy is 6d. post free. A diagram is also given in “Simple Scientific Experiments,” and one similar in “Simple Experiments in Static Electricity.” Price of either of these books is 10d. post free. To give effects of any magnitude the induction coil should be of at least 4 inches spark length capacity, one of 10 inches length capacity would give, with condensers of large size, amply large effects such as could be demonstrated in a lecture theatre. Alternating current can be used with a transformer giving about 12,000 volts at the secondary, instead of an induction coil. (2) We cannot give a formula for calculating the frequency. As far as we know it has been estimated by experiment. (3) Induction coils connected in series would not give the same effect. The discharge from an induction coil is uni-directional except for a very small reverse current which occurs. The arrangement would not be practicable with ordinary coils. A condenser used across the break in the primary of an induction coil serves a different function. It acts to minimise the spark by acting as a kind of by-pass to the self-induction current, which tries to continue along the gap between the points and further opposes the self induction current which is also endeavouring to maintain the magnetism in the core. The sparking length at the secondary terminals depends upon rapidity of de-magnetisation of the iron core. The condenser action promotes rapidity of de-magnetisation. Also its action delays the primary current at make, promoting thorough magnetisation of the core.