Archibald Montgomery Low 145 



Such apparatus has usually been constructed in two distinct 

 ways. The first is the more or less obvious method of using 

 different wave-lengths to affect different controls of, for example, 

 an aeroplane. The rudder might be turned to the right by 

 sending a radio signal of three hundred metres, or to the left 

 by transmitting a radio signal at two hundred metres wave- 

 length. The disadvantage of this method is that accurate 

 selection by an apparatus which is subject to vibration and 

 travelling at high speed, and which can be ' jammed ' by the 

 enemy, is almost impossible to-day. 



To overcome this difficulty, various mechanical selec- 

 tors have been designed. Some are operated by sound, 

 various sensitive strings being stretched to respond to the 

 different rate of vibrations, and these vibration rates 

 being transmitted by wireless, when the trembling string 

 in turn switches on power to the particular control which 

 it is designed to operate. 



The most accurate device yet discovered is based upon 

 the running of a motor at the transmitter end of the con- 

 trol and another at the receiver end, at speeds which are 

 carefully synchronized. 



This device can be readily understood by first imagining 

 a pencil which is allowed to roll down the side of a sloping 

 table. It will be found that to make this pencil climb up 

 the gradient when it is trying to run down all the time 

 it must be struck upward at a definite rate. If the 

 pencil is pushed down the table by a spring which has a 

 well-defined strength behind it, the rate at which the 

 pencil must be knocked to make it climb will vary 

 according to the natural tension of the spring. This 

 principle has been exploited in a mechanism called a 

 ' pecker/ Wireless signals are sent out from the trans- 

 mitter to the controlled aeroplane or torpedo at various 

 rates, timed by a clock device, and each of these rates 

 corresponds to the adjustment of the pecker receiving the 

 signals in the controlled mechanism. Contact by wireless 



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