Nikola Tesla Books
Books written by or about Nikola Tesla
equal to that half wave length which is obtained by computation from the velocity of light. In practice it is invariably observed that l is smaller and K is not as it should be = 1 but is often a large number, this simply following from the fact that the velocity of propagation in a circuit with considerable inductance and capacity is generally much smaller than that of light and often considerably smaller. It is to be stated that for a number of reasons it is of advantage, whatever be the actual length of the primary conductor, to arrange it so that it is symmetrical with respect to the condenser and the make-and-break device, one of the chief objects being to secure the maximum difference of potential on the terminals of the condenser. This consideration leads to the adoption of at least two condensers in series, the primary generally joining the outer coatings while the inner ones are bridged by the break device.
Coming now to the secondary quite different considerations apply. First, we must decide whether the secondary high electromotive force is to be obtained exclusively or entirely by transformation as in the commercial transformers with iron core, or not. In the first case obviously similar rules of economic design as followed in ordinary transformers will have to be respected. The secondary will have to be placed in the closest possible inductive relation to the primary and this will give an economical machine and one of relatively high frequency, since the inductances of the circuits by mutual reaction will be considerably reduced. But it is at once seen, that in a machine as here chiefly considered for the purposes followed from the outset, the connection between the primary and secondary can never be as close as in ordinary transformers, and the connection must be all the less intimate as the pressure on the secondary is increased since the wires must necessarily be placed at a greater distance from each other. From this it follows that in such a machine the free vibration of the secondary can never be quite ignored even if the electromotive force is not extraordinarily high. Now directly as the free vibration of the secondary becomes an important element to consider in the design, the careful adjustment becomes obviously imperative. It goes without saying that $! {pL \over R} $! should be as large as possible in all cases where resonant rise is one of the objects. But here is where we find in practice, and particularly in a large machine, difficulties not easily overcome. Both the inductance and capacity grow rapidly as turns are added, so much so that very soon it is found the secondary period becomes longer than that of the primary. The chief drawback is, as has been already pointed out, the distributed capacity but also the inductance though in a lesser degree. While the inductance in a certain since has a great redeeming feature and is necessary, yet it stands in the way of obtaining a very high frequency in a large machine. To get a high electromotive force we must have many turns or turns of great length and this means great inductance and this again entails the drawbacks of slow vibration. Thus, in a large machine we encounter those difficulties which meet us in the design of too large a bridge, for instance, difficulties which are based on the very properties of matter and seemingly insuperable. Make a wire rope of twice the section and it will not be able to carry a longer piece of its own, since the weight is increased in the same proportion as the section and the strain per unit of the latter remains the same. Fortunately for us in electrical machinery, of this kind at least, this limit is immensely remote owing to the wonderful properties of this agent Still the difficulties encountered on account of the capacity and inductance, and equally on account of the insulation are such as will require great deal of persistent effort to be effectively done away with in these
68
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.
