Repository: UIHistories Project: Board of Trustees Minutes - 1886 [PAGE 246]

EXTRACTED TEXT FROM PAGE:

238

6. From these averages the wools of the five breeds rank in economical value as follows, from greatest to least: Southdown, Oxforddown, Merino, Lincoln, Cotswold. 7. In point of strength, Merino wool closely approximates the average value for the five breeds considered. Its economic value would therefore be a mean between those of the Southdown and Cotswold. Comparing the relations between the total, permanent and elastic stretch produced by various strains, we reach the following conclusions : 1. The permanent stretch increases nearly as fast as the total stretch. 2. The elastic stretch increases about half as fast as the totaL 3. Consequently the elastic stretch only changes about half asfast as the permanent stretch. 4. The permanent and elastic stretch are equal, as an average, when the total stretch equals about 4.3 millimetres or 21.5 per cent, of the original length of the fibre. To better comprehend the significance of these values we may compare them with similar values for other materials the strengths of which have been determined. We may thus compare it with wood, ivory, whalebone, the metals, iron and steel, but to render this comparison more readily intelligible it becomes necessary to change the average tensile strains in grammes on fibres of wool 4 centimillimetres in-diameter to corresponding strains in pounds per square inch of section of fibre. This may be done as follows: The common diameter of fibre being 4 centimillimetres, the area of right cross section is 12.5J64 square centimillimetres. One gramme on a fibre having this area of cross section corresponds to rliih grammes per square millimetre of section, or 10 k - { | | | ™ ^ S — 0.79577S kelogramme per square millimetre of cross section of fibre. One kelogramme per square millimetre of cross section corresponds to 1422.308 pounds per square inch of section (Thurston, Materials of Engineering, I. 308). Consequently one gramme of tensile strain on a fibre 4 centimillimetres in diameter exactly equals a strain of 0.795773 + 1422.308=1131.834 pounds per square inch of section. Therefore if all the general average tensile strains for wool already found be multiplied by this coefficient we shall obtain their corresponding values in pounds per square inch. As this multiplier is a constant, it does not affect the relative values of the different kinds of wool at all. The results of this reduction are as follows: the permanent and total stretch given in millimetres, and the respective corresponding relative resistance or strains in pounds per square inch of section.

P e r m a n e n t stretch Resistance | 0.25; 0.50 ! 1.00 ! 1.50 I 2.00 ! 2.50 I 3.00 | 3.50 ! 4.00 | 4.50 5.00 "21.720 22.650 24.527 25.805!20.067 27.911 29.416 32.439 35.065 36.024141.300

I i I ! I I I ! i I

Total stretch Resistance

!

i.oo

j 21 233

2.00 3.00 24.018, 25.465

4.00 ! 5 00 26.723, 38.285

6.00 31.024

7.00 8.00 34.7361 34.801