384 Journal of Agricultural Research voi. iv. No. s 



In column 2 is given the average tension per wool fiber at the time of 

 passing the elastic limit. In column 3 is given the average tension at the 

 time of passing the elastic limit on an area of the wool fiber equivalent 

 to the area of cross section of a wool fiber whose diameter is 15/i. The 

 first two of these sheep were range-bred merinos and the third was a 

 pure-bred Shropshire. The Shropshire has much coarser wool fibers; 

 consequently the large value in column 2. But, in spite of the differ- 

 ence in breed and a large variation in the size of wool fibers between 

 Shropshires and merinos when reduced to the tension on equal areas of 

 cross section, w^e find great uniformity for the three sheep. These three 

 sheep had received similar care and feed. We should expect a greater 

 variation in elastic limit than shown above for sheep of the same breed 

 under different conditions as to feed, shelter, etc. 



DESCRIPTION OF TESTING APPARATUS 



The apparatus used in these tests consists of (i) a fiber-testing machine 

 devised for the Philadelphia Textile School and (2) a compound micro- 

 scope with micrometer eyepiece attached for accurate measuring of the 

 diameter of the fibers. 



The following description of the fiber-testing machine is given by 

 Matthews 1 (PI. LVII, fig. i and 2). 



The fibre to be tested is clamped between the jaws at (J), the pointer attached to the 

 end of the beam above the upper jaw being brought to the zero-mark on the scale (S), 

 while the lower jaw is raised or lowered in its stand until the desired distance between 

 the jaws is obtained. To obtain comparable results this distance should always be 

 the same. [We have used 40 mm. for otu" observations.] The sliding-bar (R) is 

 moved forward by turning the rod (T), which moves the rack and pinion at (P), 

 until the graduation on the wheel (G) is at zero to the indicator. Under these condi- 

 tions there is no strain on the fibre. A stretching force is then placed on the fibre 

 by moving the bar (R) backward by turning the rod (T); the motion of this bar is 

 made uniform and gradual until the fibre finally breaks under the strain thus placed 

 upon it. The graduation on the wheel (G) will then indicate in decigrams the break- 

 ing strain of the fibre being tested. The elasticity is obtained by watching carefully 

 the pointer moving up the scale of millimeters at (S) until the rupture of the fibre 

 takes place; the distance this pointer moves represents the actual stretch of the 

 fibre. . . . The weight (W) at the rear end of the beam can be moved backward or 

 forward, and is for the purpose of adjusting the balance so that there is no strain at (J) 

 when the indicator (G) marks zero. The wheel (G) is graduated in decigrams, and 

 this marks the sensibility of the machine; the total graduations on (G) running from 

 zero to 400. When fibres are tested having a greater tensile strength than 400 decigrams 

 a fixed additional weight of 10, 25, 50, etc., grams may be hung from (Wj, and this 

 must be added to the reading on the wheel when the fibre breaks. If the elasticity 

 of the fibre is so great as to carry the pointer beyond the limits of the scale at (S), 

 a shorter length of fibre must be tested. 



I Matthews, J. M. Op. cit., p. 272-274, fig. 69. 



