In instructions for making static tests the rate of application of the stress, _i.e._, the speed of the machine, is given because the strength of wood varies with the speed at which the fibres are strained. The speed of the crosshead of the testing machine is practically never constant, due to mechanical defects of the apparatus and variations in the speed of the motor, but so long as it does not exceed 25 per cent the results will not be appreciably affected. In fact, a change in speed of 50 per cent will not cause the strength of the wood to vary more than 2 per cent.[58] [Footnote 58: See Tiemann, Harry Donald: The effect of the speed of testing upon the strength and the standardization of tests for speed. Proc. Am. Soc. for Testing Materials, Vol. VIII, Philadelphia, 1908.] Following are the formulæ used in determining the speed of the movable head of the machine in inches per minute (n): (1) For endwise compression n = Z l Z l^{2} (2) For beams (centre loading) n = --------- 6h Z l^{2} (3) For beams (third-pointloading) n = --------- 5.4h Z = rate of fibre strain per inch of fibre length. l = span of beam or length of compression specimen. h = height of beam. The values commonly used for Z are as follows: Bending large beams Z = 0.0007 Bending small beams Z = 0.0015 Endwise compression-large specimens Z = 0.0015 Endwise compression-small " Z = 0.003 Right-angled compression-large " Z = 0.007 Right-angled compression-small " Z = 0.015 Shearing parallel to the grain Z = 0.015 Example: At what speed should the crosshead move to give the required rate of fibre strain in testing a small beam 2" X 2" X 30". (Span = 28".) Substituting these values in equation (2) above: (0.0015 X 28^2) n = ----------------- = 0.1 inch per minute. (6 X 2) In order that tests may be intelligently compared, it is important that account be taken of the speed at which the stress was applied. In determining the basis for a ratio between time and strength the rate of strain, which is controllable, and not the ratio of stress, which is circumstantial, should be used. In other words, the rate at which the movable head of the testing machine descends and not the rate of increase in the load is to be regulated. This ratio, to which the name _speed-strength modulus_ has been given, may be expressed as a coefficient which, if multiplied into any proportional change in speed, will give the proportional change in strength. This ratio is derived from empirical curves. (See Table XVII.)