RECENT STUDIES IN GRAVITATION. 



211 



has two ends, like a magnet, and that like poles tend to like directions. 

 Then, the couple will vanish onl}' twice in a revolution and may be 

 termed a semicircular couple. We looked for both, but it is enough 

 now to consider the possibilit}' of the quadrantal couple only. 



Our mode of working will be seen from fig. 9. The hanging 

 sphere, 0.9 cm. in diameter and 1 gm. in weight, was placed in a light 

 aluminum wire cage with a mirror on it, and suspended by a long 

 quartz tiber in a brass case with a window in it opposite the mirror, 

 and surrounded bj^ a double-walled, tinfoiled wood case. The position 

 of the sphere was read in the usual way, by scale and telescope. The 

 time of swing of this little sphere was one hundred and twenty seconds. 



A larger quartz sphere, 6.6 cm. diameter and weighing 400 gms., 

 was fixed at the lower end of an axis, which could be turned at any 

 desired rate by a regulated motor. The centers of the spheres were 



f J 4 6 6 7 



Period / 26 ^ I 



Fig. 10. — Upper curve in regular vibration. Lower curve a disturbance dying away. 



on the same level and 5.9 cm. apart. On the top of the axis was a 

 wheel with 20 equidistant marks on its rim, one passing a fixed point 

 every eleven and five-tenths seconds. 



It might be expected that the couple, if it existed, would have the 

 greatest efl'ect if its period exactly coincided with the one hundred and 

 twenty second period of the hanging sphere, i. e., if the larger sphere 

 revolved in two hundred and forty seconds; but in the conditions of 

 the experiment the vibrations of the small sphere were very much 

 damped, and the forced oscillations did not mount up as they would 

 in a freer swing. The disturbances, which were mostly of an impul- 

 sive kind, continually set the hanging sphere into large vibration, and 

 these might easily be taken as due to the revolving sphere. In fact, 

 looking for the couple with exactly coincident periods would be some- 

 thing like trying to find if a fork set the air in a resonating jar vibrat- 

 ing when a brass band was playing all round it. It was necessary to 



