Nikola Tesla Books
Books written by or about Nikola Tesla
with wire w which at the same time served to guide the ball in its up and down movement. To avoid losses the bushings did not project beyond the surface of the ball and for the same reason the cord was not fastened to a hook but a hole was drilled into the ball, the cord with a knot on the end was slipped in and a wooden plug driven into the hole, so that nothing was sticking out capable of giving off streamers or causing leaks into the air. In the first series of experiments a ball of 18" diameter was used. The ball was not perfectly round but the error due to a slight irregularity of shape was very small. The plan of connections is shown in the sketch in which the same letters are used to designate the same parts of apparatus as before. The excitation of the coil was effected by connecting the lower end to one of the terminals of the condenser - the one which was connected to that end of the secondary of the 60,000 volt transformer which was in connection with the tank. The tank, as described on a previous occasion, was usually connected to the ground but in these first experiments the ground connection was omitted to secure stronger excitation. From the terminals of the coil two thin and heavily insulated wires were led to an adjustable spark gap s which was manipulated until the maximum rise of potential on the coil was ascertained. The two sets of condensers were joined by a stout short wire W of inappreciable self-induction and resistance and the inductance of the exciting circuit was varied by inserting more or fewer of the turns of the regulating coil R into the circuit through which the condensers were periodically discharged. The wires leading from the coil to the adjustable spark gap s were, as before remarked, very thin, as short as it was practicable to make them and heavily insulated. By observing these precautions the error due to the capacity of these wires themselves was reduced to a minimum, also the loss owing to a possible formation of streamers. To reduce the capacity the wires were led far apart and then brought in line to the spark gap. The lower wire, which was connected to a point of comparatively low potential was of little consequence but on the top wire these precautions were imperative. The procedure was as follows: first the period of the coil L and capacity attached to the free terminal was ascertained by varying the capacity or self-induction, or both, of the primary or exciting circuit until resonance was reached which was evident from the maximum rise of potential. When the period had thus been determined with the capacity, say a sphere, in one position, the position of the body of capacity was varied by shifting it to another place along the wire w and the adjustment of the primary circuit was again effected until resonance was reached, generally by simply varying the length of wire of the regulating coil included in the primary circuit. Now as the self-induction of the coil L remained the same through all experiments, the apparent capacity could be easily determined from the self-induction and the known period of the primary or exciting circuit. By keeping the capacity in the primary circuit the same or, eventually, the self-induction, the procedure was simplified and the capacity in the system including coil L was then at once given by a simple ratio, as in some cases previously described. It was preferable to vary the self-induction as the change of this element could be effected continuously and not step by step, as was the case with the capacity.
The apparatus being arranged as stated, the lowest position of the ball of 18" diam., which was first used, was 9' 5" from center of ball to ground and the highest 58' 9".
To ascertain the period of the system L the vertical wire was first disconnected and only the spark wires left on, then the vertical wire was connected and the period again determined by adjusting the primary circuit, then the ball was placed in its lowest position and finally readings were taken with the ball at various heights up to the maximum elevation. The results condensed were as follows:
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October 23
In further experiments to determine change of capacity with height Tesla uses an apparatus similar to that of the previous day. As far as can be judged, the coupling between the oscillator and the measuring circuit (coil with elevated ball) was loose. The lower terminal of the latter was connected with a condenser of the oscillator circuit. Loose coupling is evidenced by the relatively weak sparks obtained across the air gap of coil L (see figure) in comparison with the sparks obtained when a similar coil was excited by the secondary of the oscillator, tightly coupled to the primary (as for example on October 4th and 5th). Under these conditions the spark oscillator would generate a single frequency, determined by the parameters of the oscillatory circuit with the spark gap.
October 23-24
He performs the experiment outside the laboratory to determine the sphere capacitance at various elevations above the ground.
The apparatus is similar to the one from the previous day and consists of the arcing oscillator and a measurement circuit. The coupling between oscillator and measurement circuit (coil with the elevated sphere) is according to our estimate small. The lower terminal of the measurement circuit is connected with one of the terminals of the oscillator capacitor. The indication of a weak coupling is the relatively small sparks in the arcing gap of coil (see figure) in comparison with sparks when a similar coil excited the oscillator secondary in strong coupling with the primary (as for example on October 4 and 5). Under these conditions the arcing oscillator operates at one frequency determined by the diameter of the oscillating circuit's coil with the spark gap.
In parallel with the coil L a vertical wire is connected with the sphere C. The sphere can be moved along the wire, and it is supported by the method shown in the picture.
The resonant state of coil L is judged on the basis of the spark across the adjustable arc-gap. This method of resonant state determination has short-comings because it requires relatively strong excitation. Strong excitation leads to current streamers which in turn change the circuit parameters. Tesla claims that current streamers change the capacitance of thin wire so that it causes effects comparable with those created by large capacitors. That is why he took precautions to reduce the occurance of current streamers when designing the apparatus. He paid particular attention to the arcing device for registration of resonance by introducing minimal capacitance in the coil circuit.
He adjusts the oscillator frequency in steps by connecting a certain number of capacitor jars (each jar had a capacitance of approximately 800cm). The fine frequency adjustment was performed by variation of the number of turns of the regulating coil. The sphere capacitance was measured indirectly. First, the resonant frequency of the coil L was determined by means of the arcing device for resonance measurement. Then a vertical wire Ï was connected along which sphere C slides. Through this method a second resonant frequency was determined. Finally, the resonant frequency was determined when the measurement circuit consisted of coil L, vertical wire Ï and a sphere C (at various heights). From three known frequencies and known coil L inductance, the sphere capacitance values are determined at various elevations.
Tesla performed a number of measurements for the purpose of checking first one then a second value of the capacitance in the primary circuit but he did not perform any calculations.
