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 galvanometer, Fig. 12. When the tube is held in the hand and subjected to a longitudinal tensile
Fig. 12.
strain, the needle of the galvanometer is deflected in one direction; 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 contact 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 transmitting 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 arrangement of the wires can be understood by the figure. Variations of
