New Technical Applications of an Electrostatic Principle


    ON May 26 Messrs. A. Johnsen and K. Rahbek, two Danish engineers, gave a most interesting demonstration to the Institution of Electrical En gineers of new electrostatic microphones, telegraphic relays, etc., based on a little-known electrical phenomenon. If a smooth plate of brass is placed on a smoothly polished slab of lithographic stone about i in. in thickness resting on a conductor, and a potential difference of 400 volts is applied between the metal plate and the conductor, a strong attraction will be developed between the plate and the stone. Messrs. Johnsen and Rahbek demonstrated that the attraction between a metal disc about 2 in. in diameter and the stone was greater than 1 kg., although the current flowing was only a few micro-amperes. Provided the disc is in contact with the stone and the microscopic current is flowing, it lifts the stone as a 1 magnet lifts its keeper. But when the current is broken the attractive force vanishes. The stone is a semi-conductor, but the voltage drop across the stone is verv small compared with the voltage drop due to the resistance of the film between the brass plate and the stone. The force, therefore, is due to electrostatic attraction, which for a plate condenser varies inversely as the square of the distance between the plates.

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    New Technical Applications of an Electrostatic Principle. Nature 107, 439 (1921).

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