114 



GRAVITATIONAL METHODS 



[Chap. 7 



A standard radio receiver and a variety of circuits and instrtiments may be 

 used for recording. With chronographs, relays must be employed; an 

 ordinary headphone receiver may be changed readily to a relay by attach- 

 ing a bridge with an adjustable contact spring to its top. In some chrono- 

 graph recorders thyratron arrangements have been applied (see Fig. 7-20). 

 3. Reception of time signals from a central station. With observatory time 

 signal reception, a pendulum station requires 24 hours, since this is the 

 interval at which these signals are usually transmitted. However, if a 

 chronometer at a central station were connected by wire to the flash box 

 at each field station, time comparisons could be made as frequently as 

 desired. Berroth was the first to apply this method in a pendulum survey 

 of a north German salt dome. For larger surveys, wire connection is 

 impracticable and radio transmission is used instead. Transmitters range 



'Z'z^, 



Fig. 7-20. Radio receiver with thyratron and mechanical relays for time-signal 

 recording (after Weber, Richter, and Geffcken). Gi , Detector circuit; Gj , amplifier 

 circuit; Ga , thyratron; Gi , mechanical relay and circuit breaker. 



in power from 50 to 200 watts and in wave length from 40 to 100 meters. 

 In the transmission of chronometer beats, the contact circuit feeds through 

 an input transformer into the grid of the modulator tube or operates a relay 

 which controls the B-supply of the transmitter. 



If a gravity pendulum is the time standard, capacitive or photoelectric 

 transmission of its beats is employed. As shown in Fig. 7-21, the pen- 

 dulum itself, or a pin fastened to its bob, is one plate of a condenser and 

 passes the fixed plate when the pendulum goes through its zero position. 

 This change in capacity may be made to control a transmitter in various 

 ways. In the arrangement shown, the pendulum passage changes the 

 tuning of a regenerative oscillator. The resulting changes in plate current 

 are amplified and operate a relay, which in turn controls the B-supply of 

 the transmitter. In the photoelectric method, the light beam reflected 

 from the pendulum mirror is used to make contact. The light source and 



