Nikola Tesla Books
Books written by or about Nikola Tesla
capacity. This likewise improves the sharpness of the adjustment. It was easy to detect variations of one sixteenth of one turn of the regulating primary coil. From the preceding data, calling l the inductance of one turn and l1 that of 22 turns, and C the capacity in the primary when 1.125 turns were used and C1 that when 22 turns were employed, we have, since the period was the same:
(l x 1.125 + connections) C = (l1 + connections) C1.
Now it is not necessary to determine C and C1 since only the ratio is needed and we may simply take the number of bottles in each case. This gives:
(l x 1.125 + conn.) x 143 = (l1 + conn.) x 17.
Now in a previous instance l and l1 were approximately: l = 4800 cm, l1 = 105,000 cm. Substituting these values we have:
(5400 + conn.) x 143 = 17 x (105,000 + conn.)
and connect $! {{(17 \times 1050 - 143 \times 54) \times 100} \over 126} $!
From this the inductance of the connections would be = 8040 cm, or 8000 cm. approx. It would be desirable, however, to eliminate the turns of the coil and so estimate the inductance of the connections directly.
Colorado Springs
Oct. 23, 1899
Experiments to further ascertain the influence of elevation upon capacity.
The coil referred to on a previous occasion was finished with exactly 689 turns on a drum of eight feet in length and 14" diam. The wire used was cord No. 20 as before stated so that the approximate estimate of self-induction and other particulars holds good. The coil was set up upright outside of the building at some distance to reduce any errors due to the influence of the woodwork. From the building extended a structure of dry pine to a height of about sixty feet from the ground. This framework supported, on a projecting crossbeam, a pulley (wood) with cord for pulling up a ball or other object to any desired height within the limits permitted and this beam also carried on its extreme end and-close to the pulley a strong glass bottle within which was fastened a bare wire No. 10, which extended vertically downward to the top of the coil. The bottle was an ordinary Champagne bottle, from which the wine had been poured out! and the bottom broken in. It was forced neck downward into a hole bored into the beam and fastened besides with a cord. A tapering plug of hard wood was wedged into the neck and into this plug was fastened the wire. The bottle was finally filled with melted wax.
The whole arrangement is illustrated in the sketch shown in which b is the bottle with wooden plug P supported on beam B also carrying pulley p, over which passes the cord for pulling up the object, which in this case is shown as the sphere C. The spheres used were of wood and hollow and covered very smoothly with tin foil and any points of the foil were pressed in so as to be below the surface of the sphere. This is a necessary
235
October 22
He applies the same measurement method as on the previous day and finds a number of values for the primary inductance and capacitance (for the purpose of determination of the inductance of the connections). The oscillator's frequency is determined on the basis of a maximum spark at the coil's terminals with 689 turns (please see the figure).
In continuing the measurements he adds a high power transformer primary coil in the oscillator's arcing circuit. He uses only some measurement data for finding the inductance of connections.
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 ω 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 ω 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.
