514 WARREN J. MEAD 



sheet. Plainly the center Hne of the rubber (the axis of torsion) 

 remains unchanged in length, but the lateral margins are stretched, 

 because the rotating of one clamp increases the distance between the 

 ends of the two clamps. This stretching is maximum at the free 

 edges of the rubber and decreases toward the center. The effect of 

 tension thus developed is illustrated in Figure 7, which shows the 

 cracks developed in the parafhn layer as a result of torsional 

 warping of the rubber. Distribution of the cracks shows plainly 



Fig. 7. — Fractures produced in parafi&n coat on rubber sheet by torsional warping. 

 (See Fig. 6 and loC.) 



the increase in the amount of tension as the margin is approached. 

 The confluent nature of the cracks, resulting in a minimum num- 

 ber of free ends, is an interesting feature. 



The rubber sheet may be so arranged that the free edges remain 

 constant in length during torsion. If the rubber sheet thus 

 mounted is subjected to torsion, there can be no change in the 

 lateral margins. There is a change, however, along the center 

 line because the turnable clamp approaches the other as it is 

 turned, thus causing shortening or compression along the center 

 line. This compression is maximum along the center line and 

 decreases to zero at the edges. A paraffin coat on the rubber 



