Ernst Ruhmer

His early work included extensive research in developing selenium cells that were more sensitive, and faster reacting, to the effects of light illumination.

In December 1902 he and Salomon Kalischer were issued German patent 151,971 for their method of producing photographic images by exposing electrically conductive selenium-coated plates.

[1] Ruhmer also designed a light-sensitive control switch, using a selenium cell, which was successfully used to automatically turn off the flow of a buoy's illuminating gas during daylight hours.

Ruhmer believed that the increased sensitivity of his selenium cells, combined with the superior receiving capabilities of professor H. T. Simon's "speaking arc", would make the photophone practical over longer signalling distances.

He reported achieving sending distances under good conditions of 15 kilometers (9 miles),[5] with equal success during the day and at night, although the longest transmissions were dependent on having clear weather.

In 1904, he was granted, along with Adolf Pieper, German patent 173,396, "A Process for Generating Permanently Undamped Electrical Oscillations", which described a method for creating "continuous wave" transmissions using a mercury-vapor vacuum-tube.

[9] This work took place during winter 1904-5, however, this approach had limited results, as he later noted that "the transmitted speech received on microphonic contact and telephone was rough and broken like that of a stammerer".

In 1900, as part of his optical wireless telephone research, Ruhmer recorded the fluctuations of the transmitting arc-light as varying shades of light and dark bands onto a continuous roll of photographic film.

In late 1909 he successfully demonstrated in Belgium the transmission of simple images over a telephone wire from the Palace of Justice at Brussels to the city of Liege, a distance of 115 kilometers (72 miles).