The physical tests are more familiar to most persons, and include (1) weight per ream, (2) thickness, (3) bursting strength, (4) tensile strength, (5) folding endurance, (6) absorption, (7) expansion.
1.—There are two kinds of paper-scales. The most common kind gives, directly, the ream weight from weighing a single sheet, and is of such convenience that almost all paper-users could well afford to have one.
Sensitive paper-scales for small samples, 4 by 4 inches in size, are of great assistance also, and should form part of the equipment of every paper-dealer.
2.—The thickness is determined by a micrometer gauge measuring to one-thousandth of an inch. In gauging thin papers it will prove more accurate to take four thicknesses, as the error in reading is thus quartered. The following table of bulks, which shows the number of pages per inch from a gauge of four sheets, will be found convenient:
| Thickness of four sheets in thousandths | Number of pages to one inch. |
| 8 | 1,000 |
| 8½ | 941 |
| 9 | 889 |
| 9½ | 842 |
| 10 | 800 |
| 10½ | 762 |
| 11 | 727 |
| 11½ | 696 |
| 12 | 667 |
| 12½ | 640 |
| 13 | 615 |
| 13½ | 593 |
| 14 | 571 |
| 14½ | 552 |
| 15 | 533 |
| 15½ | 516 |
| 16 | 500 |
| 16½ | 485 |
| 17 | 471 |
| 17½ | 457 |
| 18 | 444 |
| 18½ | 432 |
| 19 | 421 |
| 19½ | 410 |
| 20 | 400 |
| 20½ | 390 |
| 21 | 381 |
| 21½ | 372 |
| 22 | 364 |
| 22½ | 356 |
| 23 | 348 |
| 23½ | 340 |
| 24 | 333 |
| 24½ | 326 |
| 25 | 320 |
| 25½ | 314 |
| 26 | 308 |
| 26½ | 302 |
| 27 | 296 |
| 27½ | 291 |
| 28 | 286 |
| 28½ | 281 |
| 29 | 276 |
| 29½ | 271 |
| 30 | 267 |
| 30½ | 262 |
3.—Bursting strength is determined by a variety of testing-machines, constructed so as to record the pressure per square inch which may be exerted before rupturing the paper.
In a government bulletin, Report No. 89, United States Department of Agriculture, the following criticisms of this test are made: “This pressure is generally believed to represent the mean strength of the paper—that is, an average of the strength across and with the sheet. This is not true however, experience indicating that strength as thus determined more nearly agrees with the strength of the paper in the cross direction, with the minimum strength rather than with the average strength of the paper.
“Among other objections to testers of this type, is that to a certain extent the operator can influence the results at will, and even with the greatest care there is quite a wide difference between different tests of the same paper.”
4.—Tensile strength is determined by clamping a strip of paper of standard dimensions in a machine which exerts a uniform tension until the strip breaks. The breaking strength is shown on the recorder, and the amount of stretch before breaking is also registered, thus indicating the elasticity of the paper. The best known instrument of this sort is the “Schöpper,” but the machine is very costly, hence is rarely found except in well-equipped laboratories.