STRENGTH TESTING

329

with increasing force until the fibres give way and are pulled asunder. The
pressure at the bursting point is recorded on a dial by a pointer.

On a sheet of large post (i6ixi2 inches) about 12 bursts may be taken
to get a correct average. These may be 3 along each long side, 2 along each
short side and 2 near the centre. The
rubber diaphragms should not be used
too long, as they lose their elasticity,
and low readings may be expected.
Generally the difference between a
new and an old diaphragm may be

[H. E. Messmer

FIG. 141.—SCHOPPER-DALEN BURSTING
TESTER

FIG. 142.—-NEW BRITISH BURSTING TESTER,
LATEST TYPE DIAPHRAGM CONTROL MECHANISM

taken as about 10 per cent, but if possible it is desirable to check results by using
two testers. If the sample is tested on the same machine as the finished sheet
a more accurate comparison may be made.
The paper must be firmly clamped, but not so hard as to produce stress