332 MODERN PAPER-MAKING

calculation gives an average or mean breaking strain. The strip of paper,
fixed between two clamps, is used to raise a heavily-weighted lever by means
of a wheel and gearing.

The lever remains automatically fixed by a pawl at the point where the
strip is broken, and a graduated scale and pointer show the breaking strain in
pounds. In addition, the stretch which has taken place is also recorded.

Like the Mullen tester, the Schopper requires the wheel to be steadily turned,
and where great accuracy is required this may be done mechanically by a small
electric motor, or by means of hydraulic pressure. Older machines of this
type depend on a spring for the tension, and they are accurate enough on the
whole, but as the Schopper machine is the standard tester, they are falling into
disuse.

The more recent motor-driven instruments of this type, by Goodbrand—
illustrated in Fig, 144—have a two-speed gear box giving alternative loading

rates, and a graphic recorder indicating the
stress/strain diagram of the paper specimen
examined.

When using these machines which require
a strip, it will be found that strips cut in
the machine direction have a greater tensile
strength than those cut across the web. Hand-
made papers show little difference owing to
the shake being applied in both directions
during making.

While the Mullen and Schopper types do
ru * ir not ^ow results in any relation to each other,

[H. £ \kssmer - -ti i r i . ; . , . , , .

FIG. i45.-FoiDiNG TESIT* lt: w1^ be tOTD-d in practice that either machine

will bear out the evidence of the other.

Fo7<%.-Closely allied to the question of strength is that of 'folding5.
Envelope papers, paper for bags, etc., require to be capable of being folded
without danger of the paper breaking at the hinges and folds.

Folding Endurance.-Tht most widely used instrument for the determination
of folding endurance is that by Schopper (Fig. 145). A strip of paper cut to a
standard template is clamped under a known arbitrary tension and subjected to a
continuous backward and forward folding effect. This folding effect is produced
by a slotted steel plate, which through a reciprocating motion folds the strip
until fracture takes place. The number of folds required to produce fracture
is recorded on a dial, the operating mechanism of which stops at the time of
breakdown. The number of folds that a given paper will stand before fracture
occurs under a given tension offers a fair indication of its cohesive properties.