132 THE POPULAR SCIENCE MONTHLY. 



beino- reo-ulated by the screw S. The attraction between A and B 

 varie^s with the distance between them. When, in vibrating, A moves 

 toward B, the attraction rapidly increases, and B lessens its pressure 

 upon C. During a motion in the opposite direction the attraction di- 

 minishes, and B, drawn by the screw S, increases its pressure upon C. 



A similar contrivance is illustrated in Fig. 8. The diaphragm car- 

 ries an armature, A, which by its motion changes the potential of two 

 electro-magnets. These changes in magnetism cause a bar situated m 

 their magnetic field to reproduce the original vibrations. The ends of 

 the bar are held by magnetic force against two pieces of carbon, c and 

 c These and the bar are included in the primary circuit of an induc- 

 tion-coil. The resistance of the circuit decreases when the bar is drawn 

 up, and increases as the bar descends. 



The Miceophone.— The device of using several pieces of the semi- 

 conductor instead of one was early tried by Mr. Edison. He found in 

 general that the loudness of the sound was increased by thus multiply- 

 ino- the number of contact-surfaces, but also that the articulation was 

 impaired. Instruments of this nature have since become known as 

 microphones, though it is not probable that faint sounds were ever 

 augmented through their agency. Fig. 9 shows one of the first forms 

 of the microphone, invented by Mr. Edison, April 1, 1877. Four pieces 



Fig. 9. 



Fig. 11. 



of charcoal are used, G 0, etc., each supported by an upright spring, 

 as at S and .S'. The piece of charcoal nearest the diaphragm impniges 

 upon a disk of carbon, which is fastened to the centre of the diaphragm. 

 The primary wires of an induction-coil are attached to the diaphragm 

 and the spring S'. The circuit is then completed through the semi- 

 conductors. Other forms are shown in Figs. 10 and 11. ihe former 

 has two carbons, separated by a plate of metal. The latter has three 

 contiguous pieces of carbon. 



