Blake's microphone
The microphone of Francis Blake is shown in the adjacent figure in cross section. Blake invented it in 1878.
functionality
In all the microphones mentioned before Blake, one of the two pieces of coal , usually made of compressed coal , was immobile, while the other moved freely within a certain limit and under the influence of the vibrations of a membrane etc. with greater or lesser force against the attached piece of coal was pressed.
With the apparatus constructed in this way, it was difficult to bring the two contact pieces exactly into the position in relation to one another which they must assume in order to achieve a clear, flexible and distinct sound transmission.
Furthermore, such devices, even if they are properly regulated, are extremely sensitive to vibrations and atmospheric influences, so that they always need new regulation.
Blake has eliminated this disadvantage with its microphone. The same consists of a wooden housing A with a sound opening E, which leads to a membrane CC. The holders B and B 'screwed onto the wooden housing serve to hold the contact pieces which are springy against one another.
First, the platinum contact piece h is pressed slightly against the membrane by the spring g. The coal piece e is also attached by means of the spring d in such a way that it swings freely with the membrane. Under the pressure of the stronger spring d, however, it offers greater or lesser resistance to the vibrations of the membrane. However, this also changes the pressure that the two contact pieces exert on one another, and consequently also the resistance in the circuit leading via S, w, g, h, e, d, F and G.
The device can be easily adjusted by means of the lever F resilient on the spring l and the adjusting screw G and then does not require any further adjustment.
literature
- Harry Dittrich, Günther Krumm: Elektro-Werkkunde Volume 5 / Professional experience for telecommunications fitters and telecommunications mechanics. 4th edition, Winklers Verlag, Darmstadt, 1971
- The big book of technology. Publishing house for knowledge and education, Bertelsmann GmbH publishing group, Gütersloh, 1972