Nikola Tesla Books
Books written by or about Nikola Tesla
Colorado Springs
June 16, 1899
Experiments were continued today. A new ground connection was made by digging a hole 12 feet deep and placing a plate of copper 20"x20" on the bottom and spreading coke over it again, as customary. Water was kept constantly flowing upon the ground to moisten it and improve the connection but in spite of this the connection was still bad and to a remarkable degree. It is plain that the rocky formation and dryness is responsible and I think that the many cases of damage done by lightning here are partially to be attributed to poor earth connections. By keeping the water constantly running the resistance was finally reduced to 14 ohms between the earth plate and the water main. By connecting the earth plate and water main again, the lower end of the secondary being connected to the latter, sparks would again fly over the arresters. When the water main was disconnected they again ceased.
The action of the waves spreading through the ground was tested by a form of sensitive device later to be described and it was found that there was a strong vibration passing through the ground in and around the laboratory. The device was purposely unsensitive, to get an idea by comparison with former experiences in this direction. It did not respond when placed dose to the oscillator, but unconnected to ground or capacity, but responded 200 feet from the shop when connected to the ground with one terminal. It responded also all along a water main, as far as it reached, although it was connected to the ground fairly well. The action on the device was still strong when there were no sparks from the secondary terminal. This is a good indication for the investigation of waves, stationary in the ground. It was concluded the earth resistance was still too great. Possibly the ground affects the primary and the secondary, more than assumed, by the formation of induced currents.
To be investigated.
Colorado Springs
June 17, 1899
Measurements of resistance between ground wire and water main showed the surprising fact that it was 2960 ohms, and even after half an hour watering it still was 2400 ohms, but then by continued watering it began to fall rapidly. Evidently the soil lets the water run through easily and being extremely dry as a rule it is very difficult to make a good ground connection. This may prove troublesome. The water will have to be kept flowing continuously. The high resistance explains the difficulty, from a few days before, of getting the proper vibration of the secondary. The first good ground was evidently at the point where the water main feeding the laboratory connected to the big main underground and this was several hundreds of feet away. This introduced additional length in the secondary wire which became thus too long for the quarter of the wave as calculated. The nearest connection to earth was as measured about 260 feet away and even this one was doubtful.
Measurement of inductances primary, secondary and mutual induction.
Readings for two primary turns in series showed:
I = 34, E = 7, R = 0.015, ω = 716, I = $! {E \over { \sqrt {R^2 \, + \, ω^2 \, L^2}}} $!
37
June 16
June 16
In these experiments the influence of grounding and ground generally on the high frequency oscillator operation is studied. On the basis of series of experimental measurements of the resistance between water pipe and grounding plate, waves intensity which propagate around the oscillator and arcing on lightning arresters, Tesla concludes that the grounding resistance is high. Besides this ground effect he foresees that induced current in ground (as conductor) will influence the oscillator primary and secondary).
The waves which propagate through the ground are subject to Tesla's special interest. Already in 1893 Tesla established the hypothesis on the possibility of using the ground as the only conductor of energy transmission between receiver and transmitter antenna(6). Further development of this hypothesis he described in the patent "Electrical Energy Transmission System" submitted in 1897(13).
June 17
The repeated measurement of ground resistance between water pipe and grounded plate clearly shows that the terrain around the laboratory is poorly conductive. After a long period of ground moistening the resistance was reduced (probably the soil around the ground plate was moistened). The obvious ground resistance instability leads Tesla to think that grounding will cause difficulties in oscillator operation. The connection to water pipe as alternative grounding has, according to Tesla, disadvantages because this introduces the additional conductor in the oscillator circuit. Whether this is correct depends on a number of factors (e.g., on distance between water pipe and ground surface), but it is certain that the water pipe provides better grounding than the already mentioned grounding plate.*
The oscillator primary and secondary inductance Tesla measures on the basis of impedance module and known resistance. He determines the impedance from the ratio between voltage and current at a frequency of approximately 144 Hz. He indicates the coil's resistance, but he doesn't explain whether he determines them by measurement or by calculation.
Mutual primary and secondary inductance he determines from the ratio of no-load voltage in the secondary and excitation current in the primary multiplied by the circular frequency at which he performs the measurement.
The term "L-reduction" now is not clear. The term L - M2/N, corresponds to the primary inductance when the secondary coil is in short circuit. On the basis of mutual inductance and primary and secondary inductance it is found that the transformer link coefficient is approximately 0.6, which indicates a very good link.
* The experience with grounding in this period was small. On June 16 Tesla explains the grounding as "usually", and under this he probably considers the usual lightning grounding, The grounding for the purpose of telegraphy with one wire dates from 1838, when Stinheil demonstrated the use of the ground as a return conductor. In 1893 Tesla shows the plan for wireless transmission system(6), where the alternating current's source connects by "one terminal to ground (the water pipe is suitable as well) and the other terminal to a body of large surface P". The grounding is achieved by connection to a water pipe on receivers made by Popov in 1895(32), In about 1895 Marconi performs the experiments with Hertz apparatius, so that one inductor's arcing device terminal connects to ground, and the other is connected to the elevated conductor with the end capacitance(30). Detail analysis and measurements of grounding influence and types of ground were performed in about 1905 and later (please see ref. No. 19, p. 796).
