Page:Paper and Its Uses.djvu/109

Rh with a sharp knife. The piece is placed on the end of a hook, and the scale, marked for reams of 480, 500, and 516 sheets, gives the weight of a ream of demy of the substance of the pattern, and by reference to tables the equivalent weight in any other size can be found.

Size.—The size of the paper is checked, and the papermaker guarantees to be within one-half per cent. of the measurement ordered. The paper is tested at the same time for squareness.

Thickness of Sheet.—For testing the thickness of the sheet a micrometer is used. The screw micrometer is not so exact on a yielding material like paper as is the spring micrometer, which gives the reading on a dial in thousandths of an inch. Machines are obtainable in pocket or stand form, giving measurements as close as $1⁄2000$ of an inch. The micrometer serves to check the thickness of supplies of paper or cards, and to indicate the bulk of a volume consisting of a certain number of pages or leaves, enabling covers for books to be prepared before the printing is completed.

Tensile Strength and Elasticity.—These tests may be carried out on various machines. The method adopted is to cut strips of a standard width, clamp in the machine clips which are a definite distance apart, and to place the strip to be tested under tension by