330

MODERN PAPER-MAKING

on the edges of the area to be tested. The rate at which the wheel is turned
produces different results. About two to three revolutions per second is a
usual speed. A slower speed gives the fibres longer time under pressure, when
less is required to tear them apart. A uniform, not jerky, movement of the
wheel is essential. The highest bursting point of paper is reached at normal
humidity, about 5 to 8 per cent moisture. If the paper is above or below

this moisture content, lower readings
are obtained. It is as well to remem-
ber that a paper newly made may be
expected to gain up to 10 per cent
increase of strength when properly
matured. Sometimes different read-
ings will be obtained according to
whether the top or under side of
the sheet is clamped next to the
diaphragm.

The sheet may be tested both ways.
Two operators sometimes get differ-
ent results on the same sheet, but two
experienced men generally come very
close in their averages.

There are other paper testers of
this type, such as the Ashcroft and
Schopper-Dalen,but they all work on
the same principle, differing only in
structural details.

Certain modifications have been
recommended for use with the
Schopper-Dalen type of tester in the
diaphragm pressure control mecha-
nism. These are incorporated in
the more recent instruments made
by Goodbrand, of Stalybridge, the latest of which is illustrated in Fig. 142.

Whenever possible, the operator who tests the sample should also test the
finished sheets of the paper. In connection with these tests, it is necessary to
point out that there is no corresponding proportional increase of bursting
strength as the substance of the paper increases. Some paper buyers seem to
be under the impression that this is so, and we know of an instance where a
sample was sent to be matched for quality, but the paper had to be of a
much heavier substance. When the sheets were sent in, the paper was

[H. E. hlessmer.
Fie. 143.—SCHOPPER TENSILE TESTER ARRANGED FOR
ELECTRIC DRIVE