Nikola Tesla: Colorado Springs Notes, 1899-1900 Page 37

June 16

In these experiments Tesla investigated the influence of grounding* on the HF oscillator. The main point of interest for him was the propagation of electrical waves through the Earth. He had already put forward the hypothesis that the Earth could be used as one of the conductors in transmission of energy from a transmitting to a receiving aerial in 1893⁽⁶⁾. He further developed this hypothesis in his patent application "Apparatus for transmission of electrical energy" filed in 1897⁽¹³⁾.

* At this time there was relatively little experience with grounding. He explains in this entry that grounding was made in "the usual way as here practised", probably referring to lightning conductor grounding. Grounding for single-wire telegraphy dates from 1838, when Stinheil demonstrated that the Earth could be used as the return conductor. In 1893 Tesla described his system for energy transmission without wires⁽⁶⁾ where the alternating current source is connected "with one of its terminals to earth (conveniently to the water mains) and with the other to a body of large surface P''. Popov's receiver of 1895 also used rounding via a water pipe⁽³²⁾. Around 1895 Marconi did some experiments with a Hertz apparatus grounding one terminal of the inductor and leaving the other connected to an elevated conductor with a terminal capacity. Exhaustive studies of the influence of the form of grounding and the nature of the ground were made around 1905 and later⁽³⁰⁾.

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).