Chap. 7] 



GRAVITATIONAL METHODS 



105 



ferent phase, that is, a different distance from the crossweb, and "coinci- 

 dence" occurs when the pendulum image coincides with the crossweb. 

 For observation of the flashes, light is shone intermittently through a 

 diaphragm upon the pendulum mirror and thence to the telescope; the 

 diaphragm is attached to the armature of an electromagnet actuated by 

 the electric contact in the chronometer or astronomic clock. Light source, 

 electromagnet, and telescope are all mounted in one box (flash box). 



For photographic registration of coincidences, Martin^' has described 

 the arrangement show^n in Fig. 7-12. The filament of an electric light 

 bulb is projected by means of lens Li and mirror M on a slot placed in the 

 focus of the pendulum lens, Lg. From the pendulum mirror the light is 

 reflected and passes through a cylindrical lens to the photographic plate 



u4 



5/ef 



T CUckCanfKf 



M 



Ci^lindrifl Ifstr^ Pktftfrefhft Plift 



PtnJulum 



o 



Fig. 7-12. Stroboscopic photography of pendulum by comparison with contact clock 



(after Martin). 



which advances at a slow rate. The mirror M is fastened to the armature 

 of an electromagnet actuated by the chronometer contact. The flashes 

 so recorded (see Fig. 7-13) are arranged in a sine curve; one-half period 

 is the coincidence interval. The photographic plates are evaluated with 

 an accuracy of ±0.01 mm; the error in determining the coincidence in- 

 terval is ±0.03 sec. By observing a sufficient number of coincidences 

 (usually ten), and repeating the procedure after fifty intervals, the ac- 

 curacy is increased to the point where the mean error of the result is 

 ±0.0001 (sec Table 19). This corresponds to an error in T of ± 1.2- 10"^ 

 sec, or 0.1 milligal in gravity. 



In another photographic method, the pendulum oscillations are photo- 

 graphed directly on the same film with accurate time marks and (radio) 

 time signals, transmitted by a chronometer or reference pendulum. The 

 accuracy is increased if two pendulums, swung on the same support with 



"Zeitschrift flir Geophysik, 5(3/4), 148-151 (1930). 



