The values for the spiral spring—i.e. extension for a given load—having been determined by previous observations in a special apparatus, the curve obtained is at once a measure and a permanent record of these cardinal factors, breaking strain and elasticity. As with all other such instruments, the recording apparatus introduces certain errors, which, however, by careful investigation and modification in accordance with the results, have been reduced to a minimum. Nevertheless, the director, Dr. Martens, has recently adopted a simpler instrument, altogether similar in principle, but based upon a direct reading of the two movements, in which of course these errors do not appear. For the student, however, the recording instrument is the more instructive, and we have given it preference for de­scrip­tion here, more especially as no difference in essential parts is involved.

Those who wish to pursue the matter into the most interesting details of the investigations made upon the subject, are referred to the papers published by the Institute for 1885.[15]

[15] Mittheilungen a. d. Königl. Techn. Versuchsanst, Berlin.

In testing the strength of papers by this or similar machines, it is important to observe the hygrometric state of the atmosphere at the time the trials are made, as this has been found to exert a considerable influence on the results, a paper being weaker the moister the atmosphere.

The results of the tests are expressed in the following terms:—The elongation is given directly in percentage of the original length. This is uniformly taken at 180 mm., a length arrived at after laborious investigation, as minimising the errors of experiment; in other words, as giving mean value with the minimum of variation. For the breaking strain an ingenious expression has been arrived at, viz. the length of the paper which suspended vertically, with one end hanging freely, the other fixed, would determine fracture at the fixed end. As the breaking strain would vary with the thickness, the numbers obtained in {197} units of force or weight for strips of constant breadth, would need correction in order to admit of strict comparison with one another. By substituting an expression in terms of the paper itself—since a paper of greater thickness, and requiring therefore a proportionately greater force to fracture it, weighs more per unit of area, and in the same proportion—all the numbers for breaking strains are strictly comparative one with the other. In the same way also the question of width may be disregarded.

A further mechanical test, forming a part of the scheme of investigation, is the resistance of the paper to rubbing. This test is an altogether empirical one, as the following brief de­scrip­tion will show:—A piece of the paper, about 6 inches square, crumpled by successive folding in two directions at right angles, is grasped by the thumb and forefinger of each hand, at a distance of 3–4 inches apart. It is then rubbed upon itself across the thumbs a given number of times (seven is the number chosen) and held up to the light. If no holes are visible, the rubbing is repeated. The number of times necessary to repeat the rubbing until holes appear is the measure of the resistance. A sufficient uniformity in the results of this test has been observed to make it the basis of a classification of papers, in regard to their resistance to such disintegration; they are divided into the following eight groups, beginning with the lowest:—

The classification of papers on the results of these tests cannot be more lucidly given than in the following scheme, under which the results are officially recorded:—

Class.123456
a. Mean breaking length (metres) not less than600050004000300020001000
b. Mean elongation (per cent.) at fracture not less than4·5432·521·5
c. Resistance to rubbing665431

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