Nikola Tesla Books
Books written by or about Nikola Tesla
Colorado Springs
July 8, 1899
Further conclusions relative to the best working conditions and constructional features of such oscillators derived from observations made in these and previous experiments. Beginning with the primary, the capacity should, as stated before, be best adapted to the generator which supplies the energy. This consideration is, however, of great importance only when the oscillator is a large machine and the object is to utilize the energy supplied from the source in the most economical manner. This is the case particularly when the oscillator is designed to take up the entire output of the generator, as may be in the present instance. But generally, when the oscillator is on a supply circuit distributing light and power the choice of capacity is unrestricted by such considerations. In most cases the advantages secured by using a very high frequency are so pronounced that the primary circuit will have to be designed with this feature in view. The resistance of the primary circuit should be in any event as small as it is practicable to make it. I also think that generally, the inductance should be as small as practicable for that frequency which is supposed to be arbitrarily selected beforehand. When, however, the break number is comparatively small, that is, much smaller than the number of free vibrations, it is of great advantage to have the inductance great in order to give a greater momentum to the circuit and to thus enable it to vibrate longer after each break. But if the break number is of a frequency comparable with that of the free vibrations, the inductance should be as small as possible for however small it be, the circuit will generally vibrate long enough. One more reason why the inductance should not be large in such a case is that, in the primary, it is unnecessary to raise the pressure by making $! {pL \over R} $! very large. Necessarily this factor will be large in a well designed circuit, but should be so chiefly owing to an extremely small resistance and not owing to a high self-induction. By making the inductance smaller a greater capacity may be used and this will give a greater output, a feature which is sometimes of importance. Of course, as the capacity becomes large the difficulties in the make and break device increase, but with a properly designed mercury break these difficulties are in large measure overcome. I conclude from the above facts that the best way to construct a primary in such a machine is to use thin sheet of copper or at any rate a stranded cable. I have settled upon using copper sheet in the smaller machines since long ago, this giving the best result. By using sheet a very small inductance is obtained and more length of conductor can be wound on for same frequency, at the same time the opportunity for radiation is excellent and the construction is simple and cheap. For the same section sheets heat much less than cable and the difference in this respect is so marked that I have been tempted to believe that there is a special reason for it, not yet satisfactorily explained. The actual length of the primary conductor, relative to that length which is obtained by dividing the velocity of light by 2n, n being the number of vibrations of the primary per second - is of little importance since the primary is generally but a very small fraction of that length, but I believe to have observed that it is preferable, in a slight degree, to make the conductor of such a length that, if l be this length and n the frequency, 2 Knl should = v, the velocity of light, and K should be a whole number and not a fraction. At least this seems to hold good in circuits made very long, expressly for the purpose of ascertaining whether there is truth in this idea which was arrived at by considering the ideal conditions of such a vibrating circuit. In this abstract case l should be rigorously
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July 8
Detailed consideration of high frequency transformer is actually a resume of obtained experiences and theoretical thoughts. In order to achieve the goal - the machine of higher power and voltage, Tesla looks for methods to optimize all oscillator details; arcing device, primary and secondary inductances, mutual inductance and distributed secondary capacitance. In the analysis he makes the distinction between the device which would be used for high power transmission at a distance, and that one where small power is required as e.g., for transmission of messages. For the first type a high frequency transformer would be made in a similar way as for low frequencies (therefore with a strong link), and for the other type, high secondary voltage would be achieved by the use of over voltage on the secondary circuit which is poorly linked with primary circuits. In the latter case a maximum validity factor pL/R is required and minimal secondary capacitance.
By considering the oscillator operation, Tesla came to one interesting conclusion on the primary coil conductor shape. He discovered that flat cross-section conductor is better than a circular cross-section conductor, because it gets heated less under the same operating conditions. It is considered that there is some reason for that "which hasn't been satisfactorily explained as yet". As the shape of a flat conductor is not known, it is not possible to calculate the reduction factor of conductor resistance in relation to circular conductor resistance of the same cross-section due to skin effect influence.
The area of a flat conductor is always larger than the area of a circular conductor of the same cross-section and length, and the flatter the conductor is, the more that fact is emphasized. For the ratio between width and thickness of conductor of approximately 10, the conductor has approximately 1.8 times larger area, and therefore considerable resistance reduction could occur which is, if not entirely, then at least, a partial explanation of the event which Tesla discovered experimentally.
The problem Tesla frequently turns back to, related to coils, is the question of event propagation speed through the circuit. Tesla considered that in secondary circuits, where the maximum voltage at coil terminals without additional capacitance has to be produced, it is necessary to wind the coil with wire one quarter of wavelength long. This would be quite correct when a straight conductor one quarter of wavelength would be taken and be grounded at one end. When such a system would be excited, at the open end the maximum voltage would be achieved, but its magnitude, at constant excitation, would strongly depend on whether the conductor is horizontal (when radiation is small, and therefore the resonant system with high validity factor would be achieved) or vertical (when system radiates efficiently, and it behaves as a resonant damped system). When a spiral conductor as at Tesla's oscillator was applied, the radiation is very slight as with a horizontal conductor, and therefore the high overvoltages are possible, unless they are reduced by distributed capacitance. Actually, with a spiral conductor the longitudinal inductance and capacitance are increased, and consequently the current propagation speed in the coil is reduced, which requires the shortening of the wire in order to achieve maximum voltage at the coil's terminal. When a capacitive load is added at the end of the coil (e.g., the metal sphere), the coil length has to be shortened even more in order to maintain the resonant conditions in the system. Both these effects Tesla took in account when designing the secondary.
On figures 1-3 several systems are shown, by which the secondary distributed capacitance reduction is achieved. The solution with separated turns is frequently used even today (Fig. 6) when the distributed capacitance influences the circuit operation in which that coil exists.
