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THE POPULAR SCIENCE MONTHLY. 



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 aifects 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 indefi- 

 nitely increased. This fact lies at the basis of several ingenious tele- 

 phones invented by Mr. Edison. The one shown in Fig. 22 is of ex- 

 ceedingly simple construction. A globule of mercury, 31, rests upon 

 a slightly concave plate of metal. A needle from the diaphragm in- 

 dents 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 tele- 

 phone-receiver shown in Fig. 23. The globule of mercury, 31, 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 in- 

 strument known as the pile-telephone. A piece of cork, A" 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 ordi- 

 nary voltaic pile. The other terminal plate presses against the metallic 

 frame of the instrument. When the pile is included in a closed tele- 



FiG. 23, 



Fig. 83. 



Fig. 25. 



phonic 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 



