Nikola Tesla: Colorado Springs Notes, 1899-1900 Page 400

June 9-12

In his efforts to construct a sensitive detector for small signals Tesla worked out several designs making use of the thermal effect of high-frequency current. Since the energies involved are very small (according to Tesla of the order of 1 erg), receivers based on this principle would be extremely delicate.

In the archives of the Nikola Tesla Museum, Belgrade, a slide has been found which evidences that Tesla was probably preparing to file a patent on a receiver similar to that which he described in the diary the 9th of June (see drawing on p. 399). The entry for 11th June is stamped on the back "U.S. Patent Office, Nov. 15, 1902."

The basic principle of these detectors is of an earlier date. According to Fleming⁽³³⁾, Gregory carried out measurements of radiation intensity by the extension of a thin wire in 1889, and Rubens and Ritter in 1890 using a bolometer.

June 13-14

From the very start of his work on wireless transmission of signals in 1892 - 1893 Tesla advocated the use of continuous HF current, while other experimenters were working with damped impulses. The advantage of continuous currents is particularly great in the transmission of continuous signals, such as speech. The entries for the 13th and 14th of June describe two modifications of the HF oscillator which could be used for amplitude modulation. These two circuits were probably in fact the first modulators in the history of radio. It is not known whether Tesla carried out any experiments with this apparatus, but similar ideas were implemented later⁽¹⁹⁾.

Tesla's notes illustrate how carefully he studied the design, from the power supply to theoretical aspects such as the ratio of the maximum modulation frequency to the carrier frequency.

The transmitter using "controlled arc" modulation of the oscillator power described in the entry of June 14th produces amplitude modulated wave by varying the carrier power about a mean value. The modulating signal can be of low power, so that the device as a whole can also be considered a frequency-shifting amplifier.

June 15

This trial run of the new oscillator was Tesla's first step towards the implementation of his high-power generator. The secondary of the HF transformer was made conical in order to reduce the voltage between turns at the top of the coil. This feature is described as one of the alternatives in his "Electrical Transformer" patent⁽²⁶⁾. Tesla was the first to suggest using braided insulated wires instead of solid conductors in HF circuits in order to reduce eddy currents (see e.g. ref. 46, p. 60).

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

* 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