Nikola Tesla Books
Books written by or about Nikola Tesla
charging the condenser or capacity distributed along the wire. The difficulty becomes greater still when it is realized that in an economical machine the turns must be close together, which increases the drawbacks resulting from the distributed capacity. Now one way of reducing the internal capacity is to place between the turns, and in series with them, condensers of proper capacity, but this is not always practicable. This will be later considered more in detail. By such means the full rise of pressure on the terminal or terminals of the secondary may be obtained, which is impossible with distributed capacity of any magnitude. Very often only a small rise at the terminals can be obtained as all the charge remains “inside”. Now as to obtaining the required pressure by a resonant rise there are again two ways: either to place a secondary in loose connection to a primary thereby enabling the free vibration of the secondary to assert itself, or using a secondary in intimate connection with the primary and then raising the pressure by an additional coil - extra coil - or inductance not in inductive relation to the primary. The latter method I have found preferable when a very high e.m.f. is desired. More particularly for purposes of telegraphy to any point of the globe which is one of the objects, I conclude that: 1) ratio of transformation should be as large as practicable with reference to the preceding; 2) magnifying factor of coil as large as possible; 3) minimum internal capacity; 4) high self-induction in coil for sharp tuning. Experiences up to present indicate flat spiral form of coil in sections as best suitable.
Rough estimate of period of vibration to be adopted with Westinghouse transformer 40,000 - 60,000 volts.
Required: that only one turn of primary be used because of 1) high ratio of transformation to be attained and 2) facility of regulation with the Regulating coil brought from New York. Now the total output of W.E. Transformer will be, say, 50 H.P. (though the machine may be strained to many times that output). From this follows the number of jars which it will be possible to use. We have 50 x 750 = $! {1 \over 2} $! x 60,0002 x 300 x C, assuming now 150 cycles per second, a little more than is likely to be the case. From this follows C = $! {{75 \times 10^{3}} \over {36 \times 10^{8} \times 3 \times 10^{2}}} $! = $! {75 \over {36 \times 3 \times 10^{7}}} $! farad or in centimeters we would have the capacity of condenser which the transformer will be able to charge without considering resonant conditions or other causes which may enable the transformer to charge many more jars - C = $! {{9 \times 75 \times 10^{11}} \over {108 \times 10^{7}}} $! = $! {{9 \times 75 \times 10^{4}} \over 108} $! = C = 62,500 cm. total. Now taking the capacity of one jar as 0.003 mfd. or 2700 cm. this would give only $! {62,500 \over 2700} $! = $! {625 \over 27} $! = 23 jars total, or in two series 46 jars on each side of primary. The capacity of new jars will be probably 0.0025 and a correspondingly greater number may be taken. With 40,000 volts we would be able to take $! {36 \over 16} $! x 23 = nearly 52 jars total or 104 on each side. Assuming 60,000 volts and say 48 of new jars on each side this would give capacity in the primary most suitable to the transformer 24 x 0.0025 = 0.06 mfd. and the inductance of the primary being say 7 x 104 cm. The period T would be $! {{{2 \pi} \over {10^{3}}} \sqrt{{{7 \times 10^{4}} \over 10^{9}} \times 0.06} } $! = $! {12.874 \over 10^{6}} $! and n = 77,660 per second.
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July 7
For the “resonance method” Tesla envisaged two possible types of resonant transformer: one with loose coupling between the primary and secondary, and the other with tight coupling but only with part of the secondary inductance*. This latter type he protected under the patent “Apparatus for transmitting electrical energy”, for which he applied on 18 January 1902(44); a good deal of his time at Colorado Springs was spent in developing it.
His conclusions about various parameters of the oscillator indicate that he had by then gained sufficient experience to be able to design such devices with improved performance in the parameters he wanted. As the experiments proceeded he gradually increased the voltage of the LF power supply. On June 20th he had calculated with an excitation voltage of 20 kV, but he had assumed a much higher rate of charging of the condenser, so that he obtained then a greater power than now with 40kV. The difference in the number of chargings per second is nowhere explained, nor had he ever previously described how it was calculated. The first time he had probably taken it as being equal to the number of breaks on the rotary discharger, and the second time as double the mains frequency. In this light the accuracy of “the capacity of condenser which the transformer will be able to charge” is dubious. However, Tesla did not take the value he calculated as limiting the capacitance in the primary, noting that it did not take into account resonance and other factors which might enable the transformer to charge a much larger condenser.
July 7
From his writings on experiences with high frequency oscillator it can be seen how deeply Tesla got into the operation optimization of this device. He wanted to construct the oscillator of high power and very high voltage, but he had to make a compromise as far as dimensions are concerned because they are in direct relation with the voltage and in reciprocal ratio with the operating frequency. From already mentioned patented and written works (please see comment June 4) it is known why Tesla considers that it is necessary to achieve very high voltages. Explaining Tesla's ideas on wireless transmission, Fleming wrote in 1944: "It appears that Tesla considers that transmission mechanisms will be different if powers of sufficiently high level are achieved and he started the production of high frequency power of very high level, probably hoping to cause by means of it the disturbances comparable in magnitude with cosmic disturbances. Other experiments of that time were satisfied with power of several watts and they didn't want anything else then to produce very weak signals at the distance.... Tesla's vision was focused on an attempt to produce some important efforts at long distances, and he didn't succeed. In his efforts he produced almost incidently the series of devices which were successfully used by other researchers with less ambitious goals"(20).
From the viewpoint of high voltages production method Tesla makes a distinction between transformation method in transformer with a good link (similar as at transformer of low frequency) and the method with resonant transformer. For the method with strong link it is considered that it is applicable when power transmission at a distance is in and for method with resonant transformer that it is much better at low level powers. The assumption that along with application of first method due to strong link between primary and secondary the reduction of primary inductance will occur (which would according to Tesla enable the primary circuit oscillation at high frequency) is not in general case correct. As far as the influence of the secondary self capacitance is concerned Tesla is correct when he considers it is an obstacle in achieving high voltage levels, because the transformation ratio of the resonant transformer is proportional to ratio of primary and secondary capacitance (please see appendix: Tesla's Oscillator).
For "resonant method" he foresees two methods of operation of resonant transformer: one with a poor link between primary and secondary and the other with strong link, but only with a portion of the total secondary inductance*. This latter method Tesla protected by patent "Apparatus for Electrical Energy Transmission" submitted on Jan. 18, 1902(44), and he worked a great deal of the time developing it in Colorado Springs.
The conclusions on various oscillator parameters show that Tesla's experience reached such levels that he was able to design devices with improved performance characteristics on the basis of some system parameters. As experiments progress, he gradually increases the source voltage of low frequency. On June 20th, he performed the calculations for an excitation voltage of 20 kv, but he assumed a considerably higher number of capacitor charges per second, and obtained higher power than now at a voltage of 40 kv. The difference in the capacitor number of charges is not explained, and even today it is not known how it is calculated. First time it probably was assumed that the number of discharges is equal to the number of spark interruptions of the rotating arcing device (spark gap), and at another time that number is equal to double the frequency of the network voltage. On the basis of this it is not possible to rely on the accuracy of "maximum capacitance which the transformer can charge." However, by this calculation Tesla did not limit the magnitude of capacitance in the primary because he mentioned that the value so determined does not take in account the resonant and other conditions which would perhaps enable the transformer to charge much larger capacitor.
* It is easy to show that these two methods are similar. If in the second case the portion of the secondary inductance L'2 is linked to its primary so the the link coefficient is k2, and in the first case the entire secondary inductance L2 is linked with primary so that link coefficient is k1, then, for the same secondary reaction to the primary $! {k_{1} = k_{2} \sqrt{L' / L_{2}} < k_{2}} $!
