290 



Professor John H. Poynting 



[Feb. 23, 



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

 sphere, • 9 cm. in diameter and 1 gm. in weight, was placed in a light 

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



ToAjcaurv^ 



Inclined. Mirror 



Fig. 9. — Experiment on directive action of one quartz crystal on another. 



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

 and surrounded by a double-walled tinfoilcd wood case. The posi- 

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

 The time of swing of this little sphere was 120 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 centres of the spheres 

 were 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 11*5 seconds. 



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

 the greatest effect if its period exactly coincided with the 120 second 



