Materials under Combined Stress. 87 
Two specimens were not broken with highest current 
employed, giving strength greater than 
820, 762 grms./sq. mm. respectively. 
The five specimens tested at a pressure from 900-1000 atms. 
give a mean of 724 grms/sq. mm. 
Experiments on Aluminium Wires. 
It was thought desirable to extend the experiments to 
ductile materials, which might possibly behave in a different 
manner to a brittle substance like rocksa t. Lead and tin 
were first tried, but it was found difficult to obtain satisfactory 
results owing to the fact that rupture takes place gradually, 
and the result depends on the time during which the 
stress is applied. Finally, aluminium was chosen as having 
a comparatively low breaking strength, and giving consistent 
results when tested with weights. A length of hard-drawn 
aluminium wire, *48 mm. diameter, was cut into pieces a 
few centimetres long, and alternate lengths tested with and 
without pressure. The results were : — ■ 
Breaking stress without external pressure. 
13-6, 13*8, 13-6, 14-0, 13'2, kilos/sq. mm. 
Mean : — 13*6 kilos/sq. mm. 
Breaking stress under 700 alms, pressure. 
14-2, 14-2, 12-7, 12-8, 15*2 kilos/sq. mm. 
Mean : — 13*8 kilos./sq. mm. 
There is thus no appreciable difference in the breaking 
stress with and without external pressure. 
The above experiments were carried out in the Physical 
Laboratory of the University of Munich, and my best thanks 
are due to Professor Rontgen for his help in carrying out the 
work, and especially for suggesting the methods employed 
to overcome the difficulties connected with the preparation 
of the rocksalt pieces, and the electromagnetic apparatus for 
obtaining the breaking tension. 
University College, Bangor, 1907. 
