Note 16 — THE MICROPHONE.

Mr. Hughes observes on this subject that carbon is a valuable material for such purposes, since it does not oxidise, and its effects are greater when combined with mercury. He takes the prepared charcoal used by artists, brings it to a white heat, and suddenly plunges it in a bath of mercury, of which the globules instantly penetrate the pores of charcoal, and may be said to metallise it. He also tried charcoal coated with a deposit of platinum, or impregnated with chloride of platinum, but this was not more successful than the former method. If the charcoal of fir-wood is brought to a white heat in an iron tube, containing tin and zinc, or any other metal which readily evaporates, it is metallised, and is adapted for use if the metal is subdivided in the pores of charcoal and not combined with it. When iron is introduced into carbon in this way it is one of the most effective metals. The charcoal of fir-wood, in itself a bad conductor, may thus acquire great conducting power.