138 THE POPULAR SCIENCE MONTHLY. 



and the covering surrounding the drum is kept constantly moits by 

 capillary action. A metallic spring attached to the centre of the dia- 

 phragm rests upon the drum ; Avbile receiving, the drum is revolved by 

 turning the milled screw at yl. , . , • tt- oq ti.« 



A new transmitter for the motograph is shown m Fig. 29. Ihe 



point P, projecting from the centre of the 

 diaphragm, impinges upon a wrapping of 

 plumbagoed silk, covering a small drum ca- 

 pable of adjustment by a thumb-screw. 



The Carbon-Rheostat. — a very im- 

 portant application of the property pos- 

 sessed 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 

 rheo'stats whenever a resistance is to be in- 

 serted in a telegraph-line ; as, for example, 

 in balancing quadruplex circuits. 

 Fis 31 is a vertical section. It shows a hollow cylinder of vulcan- 

 ite, 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, (7, which can be raised or lowered by turmng the 



Fig. 29. 



Fig. 30. 



screw B. The carbon-disks can thus be subjected to any degree of 

 preslure at pleasure. AVhen inserted in the line, it is ^ -tte^^^^^^^^^^^^^^^^ 

 iBg no loss of time to obtain any desired resistance. The resistance 

 can be varied from 400 to 6,000 ohms. 



