THE CARBON BUTTON. 



2 5 



closed with plugs of gas-carbon, and the plugs secured by covering 

 them with sealing-wax. The carbon plugs were connected with the 

 wires of a battery in whose circuit was a galvanometei*, Fig. 12. 

 "When the tube is held in the hand and subjected to a longitudinal ten- 



Fig. 12. 



sile strain, the needle of the galvanometer is deflected in one direc- 

 tion ; when the tensile strain is changed to a compression, the needle 

 is deflected in the opposite direction. The explanation offered is that, 

 when the tube is stretched, the number of particles of powder in con- 

 tact with each other, and therefore the intensity of the current, is 

 diminished ; when the tube is compressed, the number of particles of 

 powder in contact with each other, and therefore the intensity of the 

 current, is increased. Moreover, this instrument is so sensitive that it 

 is capable of taking up the vibrations of any sound, and of varying 

 the electric current in accordance with them, so that the sound is 

 reproduced at a distance in an ordinary telephone which may be placed 

 in the circuit. If the tube is placed upon a resonating-box, the delicacy 

 is increased. The tube in such an arrangement serves as the transmit- 

 ting and the telephone as the receiving instrument. Other substances 

 may be substituted for the white silver powder with good results. It 

 is essential, however, that the substance used be not homogeneous. A 

 piece of vegetable carbon plunged when incandescent into a mercury- 

 bath, so that it becomes impregnated with particles of mercury, when 

 placed in the tube, works quite well ; pure vegetable carbon, on the 

 contrary, is useless on account of its high resistance. Another form of 

 transmitter is shown in Fig. 13. A piece of carbon, A, is hung on 

 two arms, C, by a metal pivot, and rests at one end on a piece of 

 metallized carbon, D, placed upon a piece of sealing-wax. The arrange- 

 ment of the wires can be understood by the figure. Variations of 



