SUBSTANCE AND THICKNESS 335
The Gurley densometer operates by allowing a cylinder of specified
characteristics, having a specimen of paper to seal its upper end, to fall freely
in a second cylinder containing oil. The paper is clamped between two orifice
plates having an opening of i square inch, and the inner cylinder is timed
for a given fall equivalent to a known volume of air being displaced through
the paper at the upper end. The permeability is then reported as the time
taken for the displacement of 100 c.c. of air. An improved type of this in-
strument is nowT available in which the inner cylinder is sealed at die top and
air is forced through an inner tube extending to a paper clamp at the base
of the instrument. It will be noted that the Gurley densometer does not permit
the passage of air through the paper specimen at constant pressure.
The Gurley smoothness tester is a development of the densometer. By
this device both sides of the sheet are subjected to an equal air pressure by the
equivalent of a densometer falling cylinder. The paper is supported between
two optical flat steel blocks with a centre opening connected to the falling
cylinders, so that the passage of air is outwards between the face of the sheet
and the steel block. The rate of flow of air across the sheet is determined by
inequalities in its surface, and so is a measure of smoothness and 'finish'.
The degree of sizing of paper is usually measured by the rate of water
penetration. By means of the Currier apparatus the time is determined in
which a sample of paper is penetrated by water, the sample being pkced on a
metal plate and covered with a wet felt. Liquid penetration has been followed
by a change in the light reflection on the upper surface of a paper sheet due to
a liquid penetrating from the under side, the reflection being followed photo-
electrically.
Of interest to the printer is the softness or compressibility of paper. The
Bekk hardness tester is used for observations on this property. The instrument
consists of a metal cylinder with a spherical end attached to a rod and free to
move in a vertical plane like a pendulum. The paper sample is clamped against
a hard steel plate vertically below the pivot of the rod. The metal cylinder
is withdrawn through a standard arc and after lightly inking the spherical end
is allowed to impinge on the paper sample. The area of the ink spot produced
on the paper is a measure of its compressibility.
The rebound arc of the cylinder is also measured, since this affords an in-
dication as to whether the paper will be permanently deformed and embossed,
after printing. A paper with a small rebound arc, and therefore low elasticity,
may be permanently indented during printing.
Another attempt at deriving a • numerical measure of a rather obscure
paper property is afforded by the Bekk resistance to 'picking' apparatus.
'Picking* is a difficulty experienced on printing machines where small