GRAVITATION AS AFFECTED BY TEMPERATURE. 
389 
VIII. Conclusion. 
1. Technical Summary and Suggestions .—As the experimental work has been long 
and troublesome, it may not be out of place here to summarise the more important 
technical difficulties met and overcome. To perform this research it has been found 
necessary :■—- 
(1) To obtain a sealing material for making joints between a quartz or metal fibre 
and any metal including aluminium, so that the joints should stand a temperature of 
160° C. and considerable load. No wax would do, and no ordinary solder with flux, 
but the alloy of 88 Zn/l2Sn acted perfectly. # In the final experiments a quartz 
fibre, 15 p. diam., was attached thus to brass above and silver below, the load being 
seven gm. (which is little short of the breaking load). This stood for seven months 
though subjected to lateral shock and long-continued temperature of 130° C. 
(2) To realise a system of a delicate torsion balance in a high vacuum, provided 
with an optically true window. After long trial a wax of high melting point was 
found for fastening the two windows, but, of course, no wax is vapour-free when 
heated. Yet heating is essential for the production of high vacua. The joints 
containing the wax had, therefore, to be carefully treated in the preparation of the 
vacuum. 
(3) To test the quartz fibres. These fibres, while splendid in torsional qualities, 
are most uncertain as to-tensional strength. Systematic testing was therefore always 
adopted. 
(4) To damp out tremors in a delicate, torsion system in a high vacuum while 
leaving the main (torsional) oscillation free. None of the many known methods of 
damping is here permissible. But if a chain be suspended from the torsion beam, its 
links provide rolling friction at every tremor and the damping is excellent. Without 
this chain method, such measurements as these with high vacua would appear to be 
impossible under the conditions of the experiment. 
(5) To guard a delicate torsion system from external vibrations so as to make it 
useable even in a large city. The apparatus, heavily loaded with lead, was suspended 
by steel springs and steadied on all sides by rubber bungs. The vault in which the 
experiments were made is immediately under a workshop. Yet with this system, 
continuous readings could be taken even when the lathes and anvil were in use. 
Again, the movement of the heavy masses, M, M (total weight 100 kilos.) throughout 
the experiments, caused no trouble. 
No doubt these investigations will be repeated and extended. In the general 
design I cannot suggest any improvement. The final form I used, which in type 
resembles that originated by Prof. C. V. Boys, worked excellently and was sensitive. 
I have long felt that the greatest defect, or rather weak spot, in the apparatus lies in 
* See Shaw, “Sealing Metals,” ‘ Proc. Phys. Soc.,’ January, 1912. 
VOL. CCXVI. —A. 3 G 
