334

MODERN PAPER-MAKING

if the thickness of the paper is required to give any indication of the bulk of
a book, etc.

Substance.—The substance, or weight per given area, or more commonly
the weight per ream of a definite number of sheets, is ascertained by means
of a pair of scales and weights. The paper is cut to the required size, such as
20x30 inches (double crown), 17^x22! inches (demy), i6iX2i inches (large
post), and a sheet of this size is weighed, either by a balance scale and
special weights, or in a special lever scale, which has a graduated quadrant

plate, on which is engraved the weight per
ream of 480, 500, or 516 sheets. In these
latter scales a sample sheet of any desired
size may be weighed, so that it is an ex-
tremely useful instrument for the machine-
man to have on his table, enabling him,
as it does, to cut a sheet of the exact size
of the finished paper and weigh it, thus
avoiding the necessity of calculating the
weight in the usual mill standard size,
such as double crown, etc.

It is usual in mills making paper of
many distinct sizes and substances to have
metal templates for all the more usual
sizes, so that the machineman may quickly
cut out a sheet for checking his weight.

Air Permeability—The permeability of
a paper to air is a guide to the degree of
calendering, coating, sizing, or beating.
The two most popular instruments for
this determination are the Potts perme-
ability apparatus and the Gurley denso-
meter. The former is readily constructed
from simple laboratory materials, and has
the advantage over the Gurley instrument in that the determination is made
at constant pressure. The sample of paper is held in a clamp of known dimen-
sions and air is drawn through the sheet under constant pressure by means
of a water aspkator fitted with a Marriotte tube. By measurement of the
volume of water passing from the aspirator in a given time it is possible to
report the air permeability of the paper as volume per unit time for air passing
at a given pressure through a known area. A mathematical interpretation of
the significance of observations by this apparatus has been given.

[H. E Messmer
FIG. 14.7.—MICROMETER THICKNESS TESTER