1028 Dynamic Theory. 



the Bell transmitter, the pressure in this case operating to cast into the 

 circuit, or take out of it a greater or less amount of the ether. The deli- 

 cacy of the microphone is so great that the footsteps of a fly on the 

 sounding board are heard at a distance much louder, than with the un- 



FlG. 401. 



Fig. 401. Microphone. It consists of a bottom piece and a thin upright sounding 

 board of pine. A. Button of carbon attached to the sounding board. B. Button at- 

 tached to a steel spring c which is fastened to the base board, and presses the two but- 

 tons together. R. Receiving telephone, connected in circuit with a small battery. 



assisted ear near by. The ordinary theory of the propagation of sound 

 in solid bodies, involves the hypothesis of the change in the shape 

 of the molecules under the sonorous impact. This is shown in 

 figure 402. The action of the microphone is usually explained on 

 the theory that the molecules of the carbon buttons are alternate^ 

 shortened and lengthened, thus varying the resistance to the current, 

 which passes more readily through the compressed than through the 

 elongated molecules. 



_^ _^ FIG. 402. Showing supposed change in shape of 



Q O (_) , O {_) *** molecules of solid body under the impact of sonorous 



^L -^. _. 2Z. , impulses. 



O O O C_> C_J <* a. -Normal shape of molecules. 



r\ r\ r\ f\ C\ c b. Shortening and widening under the condensed 



c. Lengthening and narrowing under the rarefied 

 FIG. 402. 



In experiments cited in Journal of Franklin Institute, volume 107, 

 page 402, a microphone was used as a transmitter, and a metronome 

 (of regular beats) for a sounder, and a Bell telephone for a receiver. 

 Take off the diaphragm and mouth-piece of the (receiving) telephone and 

 replace it with a glass plate. Then apply the ear and listen to the 

 strokes of the metronome, and mark their force, then raise the edge of 

 the plate by degrees and the sound differs but little, even when the 

 glass is on edge, so long as it touches the case; but if it is raised clear 

 from the case, no sound is heard; but touch the case again and it is heard. 



