Aug. i6, 191S Tensile Strength and Elasticity of Wool 385 



IMPROVED TESTING APPARATUS 



Because of the difficulty of making accurate readings on the scale of the 

 fiber-testing machine, the following apparatus was devised to be used 

 with it : 



(i) An illuminated scale with the lamps inclosed to prevent reflection; 

 (2) an optical lever attached to the testing machine; (3) a large plain 

 mirror; and (4) a high-power telescope. 



The instruments are so arranged that the illuminated scale, A, is 

 thrown onto the optical lever, B, by means of which it is reflected onto 

 the mirror, C, from which it is read through the telescope, D (Pi. LVII, 



fig- 3)- 



The telescope magnifies about 28 times, and the distance between the 

 scale and the optical lever is such that the total magnification is just 50. 

 In this way very accurate readings can be taken. 



To test a fiber, the machine is first balanced so that it is in perfect 

 adjustment with the optical lever attached. A fiber is then put between 

 the jaws of the machine and 10 dgm. of force applied to take out the 

 crimp or waviness, making the fiber perfectly tight. A reading is taken 

 at this point and again when 15 dgm. of force is applied and again at 

 every 5 dgm. additional until the fiber passes the elastic limit. After the 

 elastic limit is passed, force is gradually applied until the fiber breaks. 

 A portion of it is mounted on a slide and the diameter obtained under 

 the microscope by means of a micrometer eyepiece. Young's modulus 

 and tensile strength can then be determined. 



EXPLANATION OF DIAGRAMS 



Mention has been made of the great variation in the fibers taken from 

 the same place upon the sheep's body. By means of figure 3 we can 

 express these variations more definitely. If, for example, we are investi- 

 gating the tensile strength of fibers, the best value to take would be the 

 average of that found for the separate fibers. But, if observations had 

 been taken on a hundred fibers, we should not expect that average to be 

 the same as for another hundred taken from the same place. It is there- 

 fore necessary not only to take the average but the probable variation — 

 i. e., an amount such that, if added to and subtracted from, the average 

 obtained will give two numbers such that the average for another hundred 

 would have even chances of lying between these two numbers. If two 

 samples of wool from different places on a sheep, from different sheep, or 

 from the same place under different treatment have been examined and 

 the tensile strength with probable variation determined, we must have 

 a means of knowing with what certainty we may state that one is stronger 

 than the other and by how much. While the fibers which have the highest 

 average will have a probability of being the strongest, that probability 

 may be very slight and, in fact, so slight that we hardly dare make any 



