This classification is based on the results of some hundreds of observations. It is interesting to note the differences observed in the numbers for a and b according to the direction for the test in which the paper is cut, i.e. in the direction in which it was run on the paper machine, or at right angles (see Chap. XI. p. [171]). The mean ratio for the breaking lengths (strains) may be taken as 1 : 1·6, i.e. the paper is about 40 per cent.[16] weaker across the web; the elongation under strain on the other hand is about double.
[16] In the statement of results the mean of the numbers obtained in the two directions is given.
It is also of interest to note the influence of the glazing process (p. [167]) upon the quality of the paper as determined by these tests.
First, we must notice the effect of the treatment upon the substance of the paper itself. The mean reduction of thickness is 23 per cent. On the other hand, the reduction of weight, calculated per unit of surface (square metre), is 6·7 per cent., whence we may infer an increase of surface, flattening out, in the process. These quantities, but more particularly the latter, will doubtless vary with the various methods of glazing and with the materials of which the paper is composed.
The breaking length (strain) shows a mean increase of about 8 per cent.; the elongation under strain, on the other hand, a diminution of 6 per cent.
For an interesting discussion of the question of the relative strengths of machine and hand-made paper see ‘Paper,’ by Richard Parkinson.
FIG. 78.
The thickness of a paper may be determined by means of an ordinary micrometer, such as is shown in Fig. 78. The paper is placed in the jaws of the instrument, and the screw {199} advanced until it touches the paper. The thickness is then read off on the scale. Other forms of apparatus are sold for the same purpose. In making a determination of the thickness of a paper it is necessary to take the mean of a series of observations at different points of the sheet, as the thickness may vary somewhat.