Nikola Tesla Books
Books written by or about Nikola Tesla
having two brass plugs fitted in its ends. The plugs had their inner surfaces highly polished and the distance between them was from 1/8" - 1/2". The tube is illustrated in Diagram 1. in which a is the glass tube and b b1 the plugs of metal with narrow projections C C1 for support and contact, respectively. The space between the plugs was filled about 1/3 full with coarse chips of nickel. These chips were made by a milling tool or punch so as to be as much as possible equal in size and shape, this being of considerable importance for the good performance of the instrument. The plug b had a small reamed (tapering) hole h in the center extending to some distance into the plug so as to enable its being placed on a small arbor fitting into the hole and rotated by clockwork at a uniform rate of speed. In some cases when the working of the device was excellent the speed was 16 revolutions per minute. But often the instrument was rotated very much faster in which case it was merely necessary to increase the e.m.f. of the battery which was used to strain the device to the point of breaking down. A beautiful feature of this kind of device is that by regulating the speed its sensitiveness may be regulated at will and in this respect it is preferable to similar devices which are stationary, the contact after being established being broken by tapping. The device acts exactly like a cell of selenium, its resistance diminishing when the disturbances reach it, being automatically increased in consequence of rotation and separation of the chips when the disturbances cease to affect the latter. The rotation of the device replaces here the property of recovery which the selenium possesses, otherwise the similarity is complete. To insure a quite satisfactory working and permanent state I prepare this form of device in the following manner:
The glass tube, plugs and the chips to be used are first thoroughly cleaned with pure absolute alcohol and dried. Next, one of the plugs, as b, is slipped into one end of the glass tube and the required amount of chips is put in the other, plug b1 being finally inserted closing the tube nearly hermetically, but not quite so. Now the device is placed upon a cylinder of metal with a hole in the center, to allow the small part of one of the plugs b or b1 to slip in, with some space between, and permit the plug to rest upon and in good contact with the upper surface of the metal cylinder which is then slowly heated, as by being placed upon an electrical stove or a plate supported above an alcohol lamp. When the lower plug is brought to the required temperature, sealing wax is run around the rim projecting for this purpose, beyond the glass tube. The metal cylinder is now allowed to cool down slowly until the sealing wax is in some degree solidified when the instrument is turned over and placed with the other plug on the cylinder and the operation of sealing the joint between glass and plug repeated. During this preparation the chips are of course at an elevated temperature and all moisture is expelled so that, when the instrument is ready, a thoroughly dry atmosphere exists within the same, this being essential for good performance. The atmosphere is, however, at a pressure slightly below that of the surrounding air. When the device is carefully prepared it works remarkably well, and in comparative tests showed itself superior to this kind of device of the form ordinarily advocated. During a few days I carried on tests of this kind which brought out the good qualities of this kind of instrument. In one instance two of them were compared with a third device of the ordinary form in which the sensitive grains were immersed in an atmosphere considerably rarefied and contact was broken with a tapper. In all three instruments the grains of nickel were of the same size and shape. One of the terminals of each of the devices was connected to a ground wire, while the other terminals were each joined to a piece of wire extending to a small height, these pieces of wire being the same in all particulars. All the three devices were strained as far as was practicable by batteries so as to be at the point
98
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.
