232 THE NEW WORLD OF SCIENCE 



the corpuscles being negative, will be accelerated, and pulled 

 through from the filament to the outside plates at a rapid rate ; 

 whereas if the grid is made negative, the stream of bombarding 

 corpuscles will be either retarded, or shut off altogether. Con- 

 sequently, the application of a relatively feeble electric impulse 

 to the grid can be made to control and deliver powerful electric 

 currents to the plate. In this way, when the tube is used as a 

 receiving device, it is made to amplify or enlarge the received 

 electric current. It is then called an amplifier. On the other 

 hand, when used as a generating device, it can cause a rapidly 

 alternating and powerful current to flow in the generator cir- 

 cuit, under a very moderate initial stimulus. It is then called 

 an oscillator. The capabilities of these tubes are astonishing. 

 A ^eries of them is frequently used as a multiple amplifier, in 

 receiving and magnifying very faint radio signals. Each tube 

 may successively multiply the strength of the received signal 

 say ten times. A two-tube system can then amplify 100 times, 

 and a three-tube system 1000 times. Amplifiers of as many 

 as 20 tubes have been occasionally used, and 7-tube amplifiers 

 are common; but the available ratio of amplification is not so 

 high, when so long a succession is employed. 



At first sight, it might well be considered that the vacuum 

 tube was merely a laboratory device, and not a soldier's imple- 

 ment. It is fragile, delicate and easily injured. The condi- 

 tions of military service in regard to transport are necessarily 

 so severe, that any piece of military apparatus is commonly 

 required to be capable of being dropped from the back of an 

 army mule into a pool of mud and water, without suffering 

 more than temporary embarrassment. Yet, by careful design, 

 and the cooperation of experts in manufacture, the Signal 

 Corps succeeded in producing vacuum tubes that would safely 

 withstand the vicissitudes of hurried transportation followed 

 by use in dugouts, trenches or airplanes. Large numbers of 

 these tubes had to be manufactured, tested, sorted and shipped 

 to the army in France, and many failures had to be encountered 

 before success was attained; but the very great advances that 



