Nikola Tesla: Colorado Springs Notes, 1899-1900 Page 49

June 27

The transmitter (Figs. 1 and 2) and receiver (Fig. 3) having several tuned circuits, the transmitter generating several signals at different frequencies and the receiver responding only when all these signals act at the same time, were the subject of two patent applications filed 16 July 1900 (subsequently granted)⁽³⁸⁾.

This method allows much more selective reception than a single-frequency channel, and is much less sensitive to interference, and the signal can only be decoded by a special receiver. In his patent applications Tesla likens it to a lock which can only be opened when one knows the combination.

The entry of June 27th was subsequently brought in evidence in a dispute before the U.S. Patent Office about priority to the idea of a multi-frequency system⁽⁶⁸⁾. The back of the page bears the stamp "U.S. Patent Office, Nov. 1902".

June 27

The circuit of transmitter (Figure No. 1 and 2) and receiver (Figure No. 3) with more than one resonant circuit so that transmitter produces two or more signals of different frequencies, and the receiver reacts when all these signals act simultaneously, are patented in two patents submitted on July 16, 1900⁽³⁸⁾.

By use of this system, considerably better selection is possible on the receiving location than when signal with only one frequency is used. At the same time this system enables considerably better protection against outside disturbances and the signal is coded so that only special receivers can receive and decode the signal. Such system (Tesla mentioned patents) compares with a lock which can be opened only by knowing the combination*.

* On the back of the original Tesla text of June 27, 1899 there is the seal; U.S. PATENT OFFICE, Nov. 15, 1902, and text by typewriter; "Tesla Exhibit Colorado Sketch".

June 28

Tesla considered that the self-capacity of the secondary winding was proportional to the number of turns and inversely proportional to the spacing between turns, so that the ratio of the distributed capacities of the new and the old coil is N₁ d/N d₁ (N - number of turns, d - spacing between turns).

The ratio of the inductance of the secondaries with different numbers of turns he finds from the relation

!$ {{\left({N_{1} \over N}\right)^{2}} = {{\left({N_{1} \over N}\right)^{2}}{Nd \over Nd_{1}}} = {N_{1} d \over Nd_{1}}} !$

derived from the expression for an infinitely long coil, and yielding the same ratio as in the case of capacitance.

The numerical value for the capacitance of the old coil appears here for the first time, without explanation.

The receiver experiments were probably done in preparation for a patent application. Leonard E. Curtis appears a number of times as a witness to Tesla's patents (see for example refs. 8, 10), or as one of the attorneys (on many patents from 1896 on).

June 28

After four days he again continues the calculations related to transmitter. He calculates the capacitance and inductance of transformer secondary on coil frame with twenty turns on the basis of known values for a coil with 36 turns. Here it is seen that the secondary coil capacitance he considers is proportional to the number of turns and inversely proportional to the distance between turns. Therefore he obtains that the ratio between distributed capacitance of the new coil and distributed capacitance of the old coil is equal N₁d/Nd₁ (N is the number of turns and d is distance between turns).

The ratio of secondary coils' inductances at number of turns change is calculated from the relation

!$ {{\left({N_{1} \over N}\right)^{2} {l \over l_{1}}} = {{\left({N_{1} \over N}\right)^{2}}{Nd \over N_{1}d_{1}}} = {N_{1} d \over Nd_{1}}} !$

Such ratio originates from the equation for an indefinitely long coil and the end result is the same as the one for the ratio of secondary distributed capacitances change.

Numerical data for old coil distributed capacitance appears here for the first time without any explanation. With calculated distributed capacitance and inductance for new coil, Tesla determines the oscillation period for the new secondary. This period (or its reciprocal value - frequency) compares with value obtained on the basis of the condition that wirelength in the secondary coil is equal to quarter of a wavelength. Then he adds the sphere capacitance to secondary and determines the reduction of resonant frequency. He concludes that vibration is high, because then small capacitance in primary circuit is required. In that case the source does not have to operate at full power. At the end, in series with the secondary he adds "additional coil", adds together all capacitances (obviously incorrectly) and by not taking into account the inductance of "additional coil", he determines the change of secondary resonant frequency.