ON IMPACT TESTS. 21 
The recording device consists of a centrifugal pump driven from the 
main axis and supplying a column of water in a glass tube, the height 
of which is proportional to the velocity of the wheel. A scale attached 
to the tube shows the number of revolutions and K.G.M. at any instant, 
and the drop in the water column after impact shows at a glance the 
K.G.M. absorbed by the fracture of the test-piece. The error is due 
to the energy lost by the release of the catch just before impact, and 
amounted to 0°8 K.G.M. The machine is only suitable for the 60 by 10 
by 10 mm. notched bar test, and is more satisfactory for resilient than 
for non-resilient steel, as the former causes a greater depression of the 
water column. The machine is operated by hand through friction gearing. 
B. The Charpy Pendulum Hammer.—These are made in three sizes, 
but the one used by the authors has a maximum capacity of 32 K.G.M. 
and a maximum fall of 1°42 metres ; the striking edge of the pendulum 
is 2mm. radius; the supports are spaced 40 mm. apart and are not rounded. 
This machine is arranged to test the 60 by 10 by 10 mm. notched bar, 
but it can also test a tension-piece with or without notches, and record 
the energy required to break it in a single blow. The usual size of the 
test-piece is } in. in diameter, with screwed ends, and a gauge length of 
24 in. The arrangement for tension tests has been modified and con- 
siderably improved by one of the authors. 
The Charpy Pendulum. 
Method of determining Energy of Rupture. 
Fig. 8. 
To obtain the energy absorbed in the rupture of the test-piece, multiply 
the difference of height of the centres of gravity of the pendulum at the 
