ADVANCES IN SIGNALLING 237 



In order to supply electricity for the airplane set, a special 

 little dynamo machine, of the windmill type, is supported 

 underneath the fusilage, so as to be driven by the motion of the 

 plane through the air. A picture of this machine with its two 

 windmill blades appears in Fig. 9. The apparatus is tapered 

 away towards the rear in stream-line fashion, so as to offer as 

 little useless opposition to the air as possible. The interior 

 parts of one of these airplane radiogenerators are presented to 

 view in Fig. 10. These little windmills are required to rotate 

 at a nearly constant speed over a wide range of airplane 

 velocity. A well-designed fan naturally keps its speeds of 

 rotation nearly in direct proportion to the speed at which it is 

 pulled through the air. In this case, however, it ought not to 

 change speed when the airplane goes faster or slower. This 

 means that the fan has to be designed very badly, judged from 

 the ordinary standpoint. One officer very proudly declared 

 that he had devised for this purpose the worst fan design in 

 heaven or earth, in order to meet the required conditions. 

 There is, moreover, a vacuum-tube automatic compensator 

 in the ogival apex of the dynamo case, which also aids in 

 maintaining constant voltage over a wide range in speed through 

 the air. 



The airplane telephone set is also ordinarily so arranged that 

 the observer not only can telephone to his ground station up to 

 a distance of say 20 kilometers by radio, but also with the pilot 

 of the plane, sitting close to him in the next cockpit, by ordinary 

 wire methods. 



Before the Signal Corps developed this aerial telephone 

 system, each airplane necessarily became an individual fighting 

 unit with only limited opportunities for concerted action after 

 it left the ground. A squadron of airplanes might fly together 

 under the leadership of its squadron commander, and follow a 

 preconcerted procedure. If, however, any unforeseen event oc- 

 curred during the flight to upset the original plan, the comman- 

 der had very little hope of communicating a change of orders to 

 his subordinates. The airplane telephone completely altered 



