Nikola Tesla Books
Books written by or about Nikola Tesla
The total capacity Cp is = 0.154 mfd. From this calculated, and neglecting as in most cases before the reaction of the secondary, we get $! {T = {214 \over 10^{7}}} $! or n = 46,730 per second.
Observations: A spark gap being established between the free terminal of the secondary and an earthed wire, strong streamers were seen on the latter. This shows very rigorous action and demonstrates that the potential of the neighbouring parts of the ground must be considerably affected. Very strong sparks on lightning arresters as the secondary discharge is playing over the gap. This is certainly extraordinary as the ground is now excellent on the secondary. The arc, horizontally passing about 32" long is very powerful, thick and giving a vivid light, the noise is deafening. The arc passes sometimes on a downward course. Is it attraction or due to surgings of the air in consequence of violent explosions? When large balls 30" diam. are placed in the gap the spark length is nevertheless small. This shows the secondary can not supply the great amount of energy necessary for charging the large balls to full pressure. This may be due simply to the imperfect inductive connection with the primary or to the small amount of power now available from the supply transformers, as there are only two of them, and the Westinghouse transformer works only at 1/4 of the normal pressure. This would mean roughly 1/16 of its total performance. On some points of the balls small streamers are observed; must be due to roughness or points on the places. The balls will have to be gone over and all the surface polished up. It would be impossible for streamers to break out from balls of such size unless the pressure is a few millions of volts, which cannot be the case at present. A curious feature is to see the sparks deviate and follow wooden beams or planks placed nearby. I rather think this is merely due to an effect of the currents of air which are prevented from circulating freely on the side of the plank or beam. The intensity of the vibration in the primary is evidenced by sparks passing between the turns of the regulating self-induction coil in the primary. Between the beginning and end of the coil, although only a few of the turns are inserted, the sparks are sometimes 3" long. This shows a very high e.m.f. on the primary and I almost think there must be a mistake as to the pitch estimated which, judging from these sparks, would seen to be much higher. This is to be investigated closer. Experimentation shows that it is very decidedly better to adopt one turn of primary instead of two and if a lower frequency is desired rather to increase the primary capacity. With one turn the explosions are more violent and the regulation is much more convenient. In these experiments the jars do not seem to be much strained, which indicates well. At times sparks will break through inside of the secondary between the turns and to the ground. The sparks are very strong from small wires attached to the free end of the secondary more so than from thick wires. When a coil was connected to the free secondary end the vibration could not be well established, evidently the coil was âout of tuneâ and by its capacity and inductance interfered with the free vibration of the secondary. The sparks went from the gap-box to the ground though the box was well insulated; there is danger of inflaming the building by this or by the secondary sparks following the wooden structure. The experiments were continued with 7500 volts as yesterday but the working was unsatisfactory. This showed finally that yesterday one of the jars in one set was bad and there was only one set acting, the other set being short circuited; that is why an e.m.f. of 7500 volts was sufficient yesterday. The Westinghouse transformer gains in e.m.f. as jars are put on, the maximum rise seems still remote, this argues well for the economy of the transformer. The incandescent lamps are all destroyed in consequence of the intense secondary vibration, the filaments being broken by electrostatic attraction towards the glass. Lamps were spoiled at a distance of 40 feet from the secondary free terminal! This action is likely to give trouble in future experiments. A curious observation is that all horses shy. It is due to sound or possibly to current action through the ground to which horses are highly sensitive either owing to greater susceptibility of the nerves or perhaps only because of the iron shoe establishing good ground connection. I am not quite certain that the secondary vibration is fundamental although for a lower or higher tone it is too powerful. The external gaps used in some trials seem to improve the action somewhat in rendering the discharges of the primary more sudden. If time should permit the vibration will be investigated by a rotating mirror to be prepared.
99
July 23
The device for electrical wave detection which Tesla calls "sensitive device" usually is called 'koherer'. 'Koherer' consists of a tube made of an insulator with the tube terminals between which there is metal powder (metal particles).
Normally, such device has a high electrical resistance, but when a high electromotive force is applied, the resistance is quickly reduced. The process of electrical conductivity increase in a metal powder/carbon mixture when the discharge current from a Leiden jar, passes through them, was described in 1835 by Munk of Rosenschold. In 1856 Varley observed that reduction of metal powder resistance occurs during natural electrical discharges. The important step forward was made by Branly in 1890, when he observed that the electrical spark from the distance acts on metal powder and changes its conductivity. Branly performed numerous experiments with various metal powders and the change of the resistance was determined by means of a galvanometer connected in series with the battery and coherer. In 1894 Lodge* showed that metal powder conductivity could be changed by electromagnetic wave action and that represented the last step before the general introduction of coherer for the purpose of radio waves detection. From the period 1895-1896 the coherers by Marconi and Popov are known(47).
The coherer is the device which once excited, remains in the conductive state. In order to return it again in a poorly conductive state, it has to be mechanically shaken. Mechanical shaking has to be well dosaged and occurring at the right instant. One more improved method of powder decoherance was discovered by Popov, and applied on his receiver for electrical discharges registration. In 1898 Rupp(48) discovered that slow and continuous coherer rotation maintains the powder in a sensitive state. The decoherent characteristic of rotation was previously discovered - as early as 1884 by Calzecchi-Onestia(49).
Tesla mentions that he worked with the rotating coherer in the New York laboratory, and therefore it is possible that he applied the rotation for the purpose of decoheration before Rupp. In comparison with other methods of decoheration, Tesla considers that the rotation method has advantage, because the sensitive device became similar to selensel which conducts when radiation acts upon it. Also, with a rotating speed variation, the sensitivity of the device can be varied, which has advantages. For the purpose of illustration, Tesla gives data on maximum and minimum resistance of sensitive device, and that is one megohm for maximum and 300-50 ohms, or less, for a minimum value. The maximum value is related to non-excited, and minimum to an excited device.
At the end he says that the device reacts to sound from the distance when receiver is adjusted to its maximum sensitivity.
The experiments with the oscillator. In the meantime, starting from July 16, Tesla was preparing new experiments with the transmitter. lle started the testing the day before and continued on June 23. Oscillator adjustment consists of a capacitance choice in the primary circuit, and variation of regulating coil inductance which is the portion of inductance which is the portion of inductance in the primary. The oscillation period is determined from the inductance and capacitance of the primary circuit, "without taking in account the secondary reaction", and that will be approximately accurate in the extraordinary case we have discussed previously.
In the description of sparks which appear at the points under high voltage stress, there are several interesting observations. As for example, the one on current streamers which appear at voltages lower from those at which they could be expected. Tesla considers that the reason for it is that the sphere is not sufficiently polished (and he does not doubt the assumption that it is the consequence of some short impulses, the voltage of which could be considerably higher than the effective voltage in the secondary). By the observation of spark paths, he observes that some deflect and "follow" wooden beams. He checks the radius points of the apparatus where sparks appear (regulating coil, lightning arresters and so on), studies the causes and possible consequences.
He establishes the difficulties due to the oscillator action on bulbs at distances of 40 ft. from open secondary terminal as well. Quite unexpectedly he mentions strange behavior of horses nearby in the field during oscillator operation, and gives a possible explanation. Almost not interrupting the thought, Tesla returns to the question of secondary circuit oscillation method and discharge speed in the primary. He foresees the possibility of use of a rotating mirror by means of which the nature of spark discharge could be established.
* The name "coherer" originates from Lodge and designates the device Which consists of the metal pieces in such state that transitional resistance is high, and it is reduced under the influence o; electrical radiation.
