through a portion of the solid frame of the instrument. The diaphragm in vibrating varies the distance between the plates. This alters their static charge, and affects also the electric tension of the line. The resistance of a conductor is dependent upon its shape. If an isometric block of metal be drawn out into a wire, its resistance may be indefinitely increased. This fact lies at the basis of several ingenious telephones invented by Mr. Edison. The one shown in Fig. 22 is of exceedingly simple construction. A globule of mercury, M, rests upon a slightly concave plate of metal. A needle from the diaphragm indents its upper surface, and, as it vibrates, slightly alters the shape of the globule. This alteration, though exceedingly small, is sufficient to vary the resistance of the telephonic current considerably.
It is a peculiar characteristic of a globule of mercury that it changes its original shape during the passage of a current through it. Mr. Edison has made an application of this phenomenon in the telephone-receiver shown in Fig. 23. The globule of mercury, M, is placed, together with a conducting solution, in a V-shaped tube. The currents from a transmitter, passing through the contents of a tube, elongate the mercury. This agitates the liquid and vibrates the float F, which is fastened to the centre of the diaphragm.
The Voltaic Pile Telephone.—We have shown in Fig. 24 an instrument known as the pile-telephone. A piece of cork, K, fastened to the diaphragm, presses upon a strip of platinum which is attached to a plate of copper. The latter is one of the terminal plates of an ordinary voltaic pile. The other terminal plate presses against the metallic frame of the instrument. When the pile is included in a closed telephonic
circuit, it furnishes a continuous current. The strength of this current depends upon the internal resistance of the pile, and the latter is varied by vibrating the diaphragm.
A convenient and peculiar form of receiver used by Mr. Edison is
