Paradigm Lost: Nikola Tesla's True Wireless Page 13

published in Zenneck’s book, which came out in German in 1912, both well before Tesla’s paper.³³

Skin Effect

Tesla repeatedly speaks of his system of wireless telegraphy implemented by sending messages through the earth. Here he displays his ignorance of what is now referred to as “skin effect”: that alternating currents have a marked tendency to cling to the outside (skin) of conductors. Knowledge of this goes back to the work of the Englishman, Sir Horace Lamb, in 1883 and was advanced further by his countryman, Oliver Heaviside, in 1885.³⁴ The results showed that the higher the frequency in use, the greater the tendency for the current to adhere to the outside of the conductor.

It is especially puzzling that Tesla does not mention this phenomenon as he took advantage of it in arranging for photographs of himself enveloped by sparks.³⁵ The frequency of the generator he was using was such that the energy would not penetrate deeply into his body, which meant that although he might have been burnt, he would not have been electrocuted. In an 1893 lecture before the Franklin Institute in Philadelphia, he sought to explain his not being shocked with a confused discussion.³⁶

By 1919, skin effect and the concept of skin depth (the depth of penetration of the current) would have been in the better electrical engineering textbooks.³⁷ We can calculate how far a wave might penetrate into a mountain in the United States where typical soil conductivity, \(\sigma = .005\) mhos/meter and the relative permittivity, \(\varepsilon_r = 10\).³⁸ We will assume a frequency \(f = 100\) kHz. Using the standard formula for skin depth that applies when conduction current greatly exceeds displacement current,³⁹ we have

\[ \delta = \sqrt{\frac{1}{\pi f \mu \sigma}} \]

Here \(\delta\) is the skin depth and \(\mu\) is the permeability of the soil, assumed here to be nonmagnetic. The skin depth for the numbers chosen here is about 22 meters. It is virtually impossible for the signal that Tesla imagines to penetrate a mountain having these typical parameters.

Dismissal of Gliding Waves

Let us now focus on Tesla’s Fig. 13 (shown here as Fig. 8) and his accompanying discussion. At the very top of his figure Tesla has the caption, “Hertz’s waves passing off into space through the earth’s atmosphere.” To someone acquainted with even elementary antenna theory, the picture is a puzzle. It depicts what appears to be a vertical antenna fed by a generator connected between the base of the antenna and the earth. In 1919, such an antenna would likely be of small height when measured in the wavelengths in use. Using the method of images and antenna analysis dating from the turn of that century, it should have been apparent that no radiation propagates along the axis of the antenna; instead, the radiation