ADVANCES IN SIGNALLING 231 



Vacuum Tubes. One of the most wonderful devices de- 

 veloped during the last few years, in connection with radio 

 communication, is the vacuum tube. It is used as part of the 

 sending apparatus for transmitting radio messages, and also 

 as part of the receiving apparatus, as well as for various other 

 accessory purposes. Fig. 8 shows two forms of such a tube. 

 That on the right hand is structurally the stronger, and for 

 army work has now superseded that on the left hand; but, 

 in principle, they are identical. The vacuum chamber is a 

 highly exhausted glass bulb, not much bigger than a hen's egg. 

 At the center of this vacuum chamber is an incandescent fila- 

 ment in the form of a long inverted V, which receives heating 

 current through two of the four insulated base pegs in the 

 base. On each side of the filament, near to it, but not touching, 

 is a vertical metallic wire grid or grating connected with a third v 

 insulated base peg. Outside the grid are two vertical metallic 

 plates. These two parallel plates are electrically connected 

 and make permanent contact with the fourth insulated base peg. 



So long as the filament on the inside of the system is un- 

 heated, the filament, grid, and plate remain highly insulated 

 from each other, although in close proximity within the vacuum 

 chamber. No current will flow from one to another even under 

 relatively powerful voltage. They are mechanically so near, 

 and yet electrically so far. When, however, an electric current 

 is passed through the filament, so as to heat it to cherry redness, 

 the tube becomes a scene of marvellous activity, most of which 

 remains invisible to the eye. The red hot filament disengages 

 and throws out infinitesimally small particles, called corpuscles, 

 of negatively electrified substance. If the plates are connected 

 to the positive pole of a dry battery, these negative corpuscles, 

 launched from the glowing filament, will be attracted to the 

 plates, and will bombard them. In so doing, they will give up 

 their negative charges to the plates, and cause a current to flow 

 to the latter from the dry battery. The strength of this current 

 can be greatly varied by varying the charge given to the inter- 

 vening grid or metallic grating. If this grid is made positive, 



