Page:Popular Science Monthly Volume 14.djvu/150

138 and the covering surrounding the drum is kept constantly moist by capillary action. A metallic spring attached to the centre of the diaphragm rests upon the drum; while receiving, the drum is revolved by turning the milled screw at A.

A new transmitter for the motograph is shown in Fig. 29. The point P, projecting from the centre of the diaphragm, impinges upon a wrapping of plumbagoed silk, covering a small drum capable of adjustment by a thumb-screw.

—A very important application of the property possessed by semi-conductors of changing their resistance under varying pressure, is shown in Fig. 30. The cut represents the new Edison carbon-rheostat. The instrument is designed to replace the ordinary adjustable rheostats whenever a resistance is to be inserted in a telegraph-line; as, for example, in balancing quadruplex circuits.

Fig. 31 is a vertical section. It shows a hollow cylinder of vulcanite, containing fifty disks of silk that has been saturated with sizing, and well filled with fine plumbago and dried. These are surmounted by a plate of metal, C, which can be raised or lowered by turning the

screw D. The carbon-disks can thus be subjected to any degree of pressure at pleasure. When inserted in the line, it is a matter involving no loss of time to obtain any desired resistance. The resistance can be varied from 400 to 6,000 ohms.