Prof. W. Siemens on Telephony. 95 



An essentially different path was struck out by Edison (as 

 it appears, simultaneously with. Bell). He uses a galvanic 

 series, which sends a constant current through the conduction. 3 



At the sending end a layer of powdered graphite, which is 

 gently pressed between two metal plates insulated one from 

 the other, is inserted in the circuit. The upper plate is fas- 

 tened to the vibrating membrane, and presses the graphite 

 powder more or less together in correspondence with the air- 

 vibrations. By this the resistance of the graphite to conduc- 

 tion is correspondingly varied, and thereby sinusoid varia- 

 tions, equivalent to the air-vibrations, are produced in the 

 intensity of the current passing through the conducting line. 

 As receiving-apparatus, Edison uses no membrane, but another 

 and quite peculiar contrivance. It is based on the experience 

 that the friction between a piece of metal and a paper band 

 saturated with a conducting fluid and pressed against the 

 metal is diminished when a current passes through the paper 

 to the metal. I have verified this remarkable phenomenon for 

 the case in which the direction of the current is such that 

 hydrogen is separated at the metal plate, or when the metal is 

 not oxidizable. Hence the lessening of the coefficient of fric- 

 tion by the current evidently proceeds from electrolytically 

 generated gases deposited on the plate of metal. Surprising, 

 however, remains the almost instantaneous rapidity with which 

 the effect takes places even with very feeble currents. 



Now Edison attaches the metal plate, pressed against the 

 moist paper, to a sounding-board, and draws the moist paper, 

 which is carried over a roller, under the metal piece by con- 

 tinual rotation of the roller. If now the metal piece and the 

 roller (also of metal) be inserted in the galvanic circuit, the 

 variations produced in the current by the greater or less pres- 

 sure of the graphite powder effect equivalent variations of the 

 friction-coefficient between the metal plate attached to the 

 sounding-board and the paper, whereby the former is put into 

 corresponding vibrations, which are communicated to the 

 sounding-board, and through this to the air. 



Edison's telephone is very remarkable on account of the 

 novelty of the expedients employed in it ; but it is obviously 

 not yet complete for practical use; while Bell's telephone 

 has, in its remarkably simple form, been widely spread in a 

 short time, especially in Germany ; and already much mate- 

 rial of experience has been accumulated for judging of its 

 usefulness. Its principal defect consists in the feebleness of 

 the reproduced sounds, which, in order to be distinctly under- 

 stood, require the sound-aperture to be pressed to the ear, 

 and at the other end an immediate speaking into it. Per- 



