THE AID OF THE ACHROMATIC FRINGES. 



21 



three and four turns of the screw, per turn, 



Ae = 6, 5, 5, 6, 5, 

 in successive separate experiments. Thus, mean Ae=5-5 and 



10. The same. Continued. The last experiments were made with a brass 

 rod, shouldered as indicated in figure 76, the large end (0.25 inch in diameter) 

 being threaded and screwed into the cross-piece A. The dimensions of the 

 thin part were: length L = 1.8 cm., diameter = 0.22 cm., A =0.038 cm. 2 The 

 results are shown in figure 18 (i), where A^/AA^Xio 3 = 2g. The mean rate 

 per turn is Ae = ^.g and io~ n E=2g. 2/4. 9 = 6.0. 



The end of the same rod was now ground flat again with the carborundum 

 plane. The results in figure 18 (2) are often coincident in outgoing and return 

 series, but show slight hysteresis. The mean rate (A<?/A./VXio 3 = 26 here) 

 is A0 = 3.6 per turn and lo- 11 ^^ 26. 2/3. 6 = 7. 3 the highest value yet found. 

 Either the regrinding or the gradual hardening of the rod has been effective. 



It seemed therefore worth while to further decrease the section. This was 

 done, the dimensions being, L=i.8 cm., diameter 0.175 cm., ^=0.0199 cm. 2 

 The results are given in figure 19, where A^/AA/'Xio 3 = 26.o and therefore 

 E=sXio il /Ae. The graphs have been separated for clearness. The rates per 

 turn lie between Ae= 5.6 (returning) and Ae = 6.i (outgoing), so that EX icT 11 

 is between 8.9 and 8.2, respectively. This is so near the normal value for 

 brass that a further decrease of section of the rod figure 76 was undertaken. 

 The final dimensions were L= 1.8 cm. diameter =0.138, A =0.015 cm - 2 The 

 results are given in figure 20, care having been taken not to overstrain 

 the thin rod. Here Ae/ANXio z = 25.8, Xio- n = 66.4/A^ ( and Ae per turn 

 lies between 6 . 7 (outgoing) and 8 . i (returning) . Hence E X i o~ u = 9 . 9 and 8.2, 

 respectively, so that the normal modulus of brass has actually been reached. 



11. Glass. The glass rod tested was L = 2.33 cm. long, 0.37 cm. in diameter, 

 so that A =0.107 cm - 2 Hence, since Ae/AA T Xio 3 = 16.7 here, 



The first series of results is shown in figure 21 (i). Both branches are prac- 



tically parallel, whence per turn A<? = 7.6 and EXio n = 1.02. Notwithstand- 

 ing the smooth results, this datum is about five or six times too small. Seven 



