Action of the Microphone . 47 



more difficult to adjust, and the expansion by heat of the screw 

 causes a varying pressure. It is exceedingly simple, however, 

 easily made, and illustrates the theoretical conditions better 

 than the balanced lever I have adopted in practice. 



In order to study the theoretical considerations, and that with 

 the most simple form of microphone freed from all surrounding 

 mechanisms, let us take a flat piece of charcoal 2 millims. thick 

 and 1 centim. square, and, after making electrical contact by 

 means of a copper wire on the lower surface, glue that to a 

 small resonant board or, better for the purpose of observation, to 

 a block of wood 10 centims. square. Upon this superpose one 

 or more similar blocks of charcoal, the upper surface in com- 

 munication with a wire, the lowermost surface resting flat, or as 

 nearly so as possible, on the lower block. 



The required pressure is 

 put on the upper block ; and 

 while in this state the two 

 may be fastened together 

 with glue at the sides, or, 

 better, by an insulated screw. 



The pressure can then be 



removed, as the screw or LH 

 glue equally preserves the 



force. Let the lower piece be called A and the upper B : 

 when we subject this board to sonorous vibrations, we can- 

 not imagine an undulatory movement of the actual wave- 

 length in such a mass, that is a length comparable with the 

 real wave-length of the sonorous wave, which may be several 

 feet. Nor can we imagine a wave of anv leno-th without 

 admitting that the force must be transmitted from molecule to 

 molecule throughout the entire length : thus any portion of a 

 wave, of which this block represents a fraction, must be in 

 molecular activity. The lower portion of the charcoal A, 

 being part of the block itself, has this molecular action 

 throughout, transmitting it also to the upper, block. How is 

 it that the molecular action at the surfaces of A and B should 

 so vary the conductivity or electrical resistance as to throw it 

 into waves in the exact form of the sonorous vibrations ? 

 It cannot be because it throws up the upper portion, making 

 an intermittent current, because the upper portion is fastened 

 to the lower, and the galvanometer does not indicate any in- 

 terruption of current whatever. It cannot be because the 

 molecules arrange themselves in stratified lines, becoming 

 more or less conductive, as then surfaces would not be re- 

 quired — that is, we should not require discontinuity between 

 the blocks A and B ; nor would the upper surface be thrown 



