Chap. 7] 



GRAVITATIONAL METHODS 



115 



the photo cell shown m Fig. 7-22 are in the focal plane of the lens attached 

 to the front of the pendulum receiver. When the pendulum passes through 

 the rest position, the photoelectric cell receives a light flash and passes 

 current, which is amplified and operates the transmitter through a relay. 

 Fig. 7-23 shows a photo cell connected to a four-stage resistance-coupled 

 amplifier and a transmitter without relay. 



Ctntral 

 w) hndulum 



Fig. 7-21. Transmission of pendulum beats from central station by capacitive 

 method (adapted from Mahnkopf). 



Radio time signals may be __— ^===4(.OJ PI. Cell 



picked up at the field stations 

 by standard short-wave re- 

 ceivers provided with some 

 sort of a recording device in 

 the output stage so that the 

 signals maj'' be photographed 

 on the same film with the os- 

 cillations of the field pendu- 

 lums. A simple recording 



device may be made of a telephone receiver (2-4000 ohms) by removing 

 the diaphragm and replacing it by a steel reed with a mirror. To reduce 

 static and other interference, the reed should be tuned to the signal fre- 

 quency. In the Askania mirror device (Fig. 7-24), an armature with mir- 

 ror is so suspended between the poles of a horseshoe magnet that it adjusts 

 itself parallel to the lines of force and is deflected as plate current passes 

 through the coils fastened to the one pole piece. A regular oscillograph 

 coupled to the output tube by a step-down transformer is likewise ap- 



FlG. 



Liqht Sourxt 



7-22. Arrangement for photoelectric 

 transmission of pendulum beats. 



