Still more elaborate appliances are available in which the sheet of paper is brought between two horizontal surfaces, the upper one being connected to a dial which registers the distance between the two plates and thus measures the thickness of the paper.

(3) Schopper's gauge is a convenient apparatus which registers upon a dial the exact thickness of a single sheet in thousandths of an inch or in millimetres. (Fig. 85.)

(4) One convenient form of micrometer which will be appreciated is Ciceri Smith's patent micrometer, which has several commendable features. (Fig. 85A.) The measurements are automatically recorded by an index which operates like a cyclometer, so that the thickness can be read off easily. The pressure on the is always uniform, as the instrument paper is constructed so that directly the pressure reaches a certain fixed amount the spindle ceases to advance when the head is turned. These are obvious advantages in any instrument for measuring the thickness of paper.

Fig. 85.-Dial Gauge for measuring the Thickness

of Paper.

Fig. 84.-Gauge for measuring the Thickness of Paper.

(5) In the absence of any special apparatus the paper must be folded over several times and the thickness determined with a pair of small callipers and a fine scale showing an inch divided into fiftieths, or one-hundredths.

The thickness of the paper is an important element when the question of the bulk is considered in reference

required for a book ought to be over-estimated rather than under-estimated, as it is easier to compress the papers slightly to fit the cover than to correct the mistake of using a cover which has been made too large through underestimation.

Strength. The strength of a paper may be recorded in several ways. It may be measured in terms of the weight required to break a strip of given width and length, or in terms of the pressure required to fracture a sheet fixed

horizontally between two suitable clamps the former being regarded as the breaking strain and the latter as the bursting strain.

Of the machines which register the breaking strain by means of tension applied to the strip of paper held vertically, or horizontally, between two clamps there are several well-known types.

The Schopper machine.-This is the recognised standard machine which has been devised to meet the requirements of the Government paper-testing institutes in Germany. The strip of paper cut to a definite width of 15 millimetres (fiveeighths of an inch) and a length of 180 millimetres (seven inches) is fixed vertically between two clamps and submitted to tension, which is produced by the movement of a heavy weight actuated at a uniform rate. The instrument also records the amount of elongation which the paper undergoes before it breaks. When the paper breaks the pointers remain stationary so that the readings of breaking strain and elongation can be noted.

The operation of the machine (see Fig. 87) is as follows: The strip of paper cut to the proper length and width is inserted between two clamps E, E. When

the handle is turned the clamp E, is drawn slowly downwards, and this movement causes the lever A to move gradually outwards along the segment B by means

of a rack work G, which operates on the small segment Cattached to the upper end of the lever. When the paper breaks the lever A remains stationary, being held in position by lugs which drop into small notches cut on the segment B.

The stretch of the paper while under tension is registered by means of a

lever H, the indicator of which measures the stretch on a small segment attached to the lever A.

The Leuner machine. The paper cut to a standard size of 15 millimetres wide and 180 millimetres long, is inserted horizontally between two clamps. The tension is applied through a spiral spring actuated by a hand wheel or by means of a small water motor. The breaking weight and elongation are automatically registered on a piece of squared paper. (Fig. 88.)

The Wendler machine.-This is a convenient apparatus in which the tension is produced by means of

a spiral spring, and the elongation measured by a slide and pointer.

is

This machine simple in construction, the paper being stretched horizontally between two clamps. By the movement of the handle one clamp is drawn back and the tension applied to the strip by means of the spiral spring attached to the second clamp.

The Marshall machine. -This finds considerable favour in England because it is a cheap, portable and convenient apparatus. The width and

length of the strip can be varied, and this is a special feature of the machine, which proves very useful for certain investigations. The elongation of the paper under tension can be measured.

The paper which is to be tested is fastened between the two clips AA. As soon as the screw B is turned to the right, a pressure is put on the spring C (through the straining of the paper), which is connected with the spindle D, the joint E and the segment which gears into the pinion, working the hand, and registers the breaking strain in pounds on the dial.

The spiral spring G serves as a stop for the spindle D, so that it stops exactly on the spot to which the pressure on the spring C has brought it.

By turning the screw B to the left, the hand moves back to zero (the startingpoint). The plate A indicates the stretch of the paper.

The Carrington machine.-One of the early forms of paper-testing machines. The width of the strip can be varied from inch to 2 inches. It is used by H.M. Stationery Office for testing brown papers. (Fig. 93.)

The Jackson machine. This has been introduced for testing strips up to 4 inches in width. It is simple in construction, consisting of an ordinary spring balance mounted on a stand, and provided with clamps for the paper. (Fig. 94.)

In testing paper for the breaking strain, several interesting questions arise. (1) The necessity of selecting some standard length for the test strip. It is clear that the selection of a short strip tends to give a high reading, while a long strip may be expected to give a low reading. For example, a strip 12 inches long will give a lower result than the average of four tests on strips each 3 inches long. If the 12-inch piece breaks at 16 lbs., the strength of the paper is taken