On a Quartz-thread Gravity Balance. 



123 



electrode is positive, then the small velocity of the positive ions is not 

 favourable to their being dragged away from the electrode before they 

 can recombine, so that the current is very small unless a very great 

 E.M.F. is applied. 



* On a Quartz-thread Gravity Balance." By Kichard Threlfall, 

 lately Professor of Physics in the University of Sydney, and 

 James Arthur Pollock, lately Demonstrator of Physics in 

 the University of Sydney. Communicated by Professor J. J. 

 Thomson, F.K.S. Eeceived April 11— Eead April 27, 1899. 



(Abstract.) 



The first part of the paper contains an account of the instrument in 

 its present form, an account of the investigations leading up to the 

 form adopted being relegated to an appendix. 



The principle of construction is as follows : — A quartz thread 

 (which requires to be prepared with much care) is stretched horizon- 

 tally between two supports, to which it is soldered. At one end the 

 point of attachment is the centre of a spring of peculiar construction, 

 designed so as to be capable of displacement in the direction of the 

 thread, but incapable of any transverse motion or vibration. 



At the other end the thread is attached to the axle of a vernier arm 

 moving over a sextant arc ; by turning the axle the thread may be 

 more or less twisted, the amount of twist being ascertained in terms of 

 the divisions of the sextant arc. 



Midway between the two supports the thread is soldered to a short 

 length of fine brass wire, which is adjusted so that the centre of 

 gravity of the wire does not lie immediately above or below the thread, 

 but at some distance from it. The wire forming the " lever " is then 

 rotated about the thread as axis in such a manner that the two halves 

 of the thread are twisted through about three whole turns, and the 

 torsion of the thread is then of such a value that the lever assumes a 

 horizontal position. This adjustment is made by weighting the lever 

 with a small speck of fusible metal. The " balance," which determines 

 the position of the lever with respect to the horizontal plane through 

 the thread, is composed of the earth's gravitational force on the one 

 hand, and the forces of resilience of the twisted thread on the other. 

 Were gravitational force to increase, the centre of gravity of the lever 

 would fall, the end of the lever would move out of its sighted posi- 

 tion, and the thread would have to be slightly twisted by the vernier 

 axle in order to bring the lever back to its sighted position. 



Differences in the gravitational intensity at different stations are 

 expressed in terms of the amount by which one end of the thread 



