Dr De Forest's Speaking Flame -- July 21, 2023

Philadelphia Inquirer, 15-July-1923

Inventor Lee De Forest explains a "flame microphone" which he planned to use with his Phonofilm sound picture process.

Dr De Forest's "Speaking Flame"

One of the most interesting of recent scientific developments and one which will appeal to the public at large, is the "speaking flame" which Dr De Forest has invented and will make use of in his Phonofilm or speaking pictures. In describing the new idea, Dr De Forest says: "Take the ordinary bat-wing gas burner or a certain form of Welsbach mantel gas light, or special forms oxy-acetylene gas flames, insert two heat-resisting electrodes therein, in proper relation to the flame and to each other, connect these electrodes to an appropriate electro-motive force, and you will then have an extremely sensitive sound converter which gives an electric reproduction of the sound waves in the air enveloping the flame, which is of an entirely different order of fidelity from that ever obtained from any form of microphonic device, using a diaphragm, whether this be of the carbon, electro-magnetic, of electrostatic variety. In my phonofilm work we have found in the same way that when a series of very fine and very short platinum wires are heated to a dull red from a local source of current the resistance of these wires changes, alternately increasing and decreasing in conformity with the sound waves impinging thereon: so that from a telephone transformer connected in series with a battery and this thermo-microphone, a remarkable faithful representation of the sound waves is obtained, even though the frequency of these be as high as 3000 per second. The sensitiveness of this device is greatly enhanced through a gentle stream of air, by fluid evaporation in the neighborhood, or by other auxiliary means. Of all the diaphragm types of transmitters, unquestionably the electro-static type, as perfected by the engineers of the Western Electric Company, comes nearest to approximate perfection. While this is extremely insensitive compared with the best carbon microphone types, there is no comparison between the fidelity of reproduction by the two means. But one listening in a telephone to the reproduction by means of the flame microphone, and then by means of the electro-static microphone will at once exclaim that the fidelity of reproduction in the first case is of quite a different order from that obtained even from the highly perfected diaphragm of the best electro-static microphone."

Film Daily, 03-July-1923

Brewster color was a relatively inexpensive two-color process.