120 



GRAVITATIONAL METHODS 



[Chap. 7 



P 



L 

 6L *-^ 



^-^' 



Since the flexure of the support produces an increase in period, the 

 flexure correction (sometimes called only flexure), is always negative. 

 Flexure is determined experimentally (1) by applying an external force 

 (producing a deflection of the pendulum apparatus), or (2) from the 

 displacement caused by the moving pendulum itself. Since (according to 

 eq. [7-26a]) the elasticity of the support is given by the ratio of horizontal 

 force and corresponding displacement, the horizontal stress may be applied 

 statically to the pendulum receiver by weights and pulley and the resulting 



displacement may be measured by a micro- 

 scope or an interferometer. The external im- 

 pulses may also be produced periodically; then 

 the forced oscillation amplitude of a light aux- 

 iliary pendulum (at rest at the beginning of the 

 test) is measured. 



Methods employing external forces are now 

 superseded by those making use of the effect 

 of the oscillating pendulum itself. They meas- 

 ure the corresponding displacement of the re- 

 ceiver or the effects of the "driving" upon a 

 companion pendulum. For making direct flex- 

 ure observations, the pendulum apparatus is so 

 set up that the pendulum swings to and fro in 

 respect to the telescope of the flash box. A 

 mirror, P (see Fig. 7-26a), is attached to the 

 head of the pendulum receiver and an inter- 

 ferometer is placed between pendulum and 

 flash box on an independent support. From a 

 source, SL, of monochromatic light (sodium- 

 chloride in an alcohol burner) the light beam 

 travels through the lens, L, to the two plane- 

 parallel glass plates, S and C. >S is provided 

 with a semireflecting backing of silver so that 

 part of the light is reflected to the mirror, P, 

 and from it into the telescope, T, while another 

 portion goes through the plates, S and C, to a stationar}'' mirror, M, and 

 thence into the telescope. 



The glass plate, C, is a compensator to make the two light rays travel 

 through exactly the same substances and the same thicknesses on their 

 way to the telescope where they unite under conditions producing inter- 

 ference. As shown in Fig. 7-266, a number of dark and light bands appear, 

 the former corresponding to a phase difference of one-half wavelength and 

 the latter to a full-wave phase shift. When the pendulum mirror, P, 



Fig. 7-26a. Interferometer 

 (after Wright). 



Fig. 7-266. Fringes and 

 fringe displacements (after 

 Wright). 



