THE AID OF THE ACHROMATIC FRINGES. 55 



Size of fringes is controlled by rotation around the vertical. The iron bricks 

 A,A',B were each 10X2X3.5 cubic inches in my apparatus, certainly rigid 

 enough for the purpose. The inner faces should be smooth and parallel. 

 Between them at the top the wooden bar DD has been bolted in place which 

 carries the rods of the railing a a', etc., with clamps for the support of the 

 forks bb' of the pulleys cc'. It also carries the standards dd' and arms ee' t 

 to which two silk threads //' are adjustably attached for the bifilar support 

 of the bar FF', when not in use. 



The stresses on the system thus seem balanced throughout, the ultimate 

 tendency being a compression of A and A' in a horizontal direction, which 

 is of course negligible. The only discrepancy which may be effective is the 

 possible flexure of the plate C by the varying weights ww'. It is for this 

 reason that the weight of A,A',B was made excessive as compared with ww. 



29. Preliminary observations. The first experiments were made with 

 hard-rubber tubes each L = 2.2 cm. long and A = 1.46 cm. in sectional area. 

 The offset 55 (fig. 30) was not at first applied and the beam F showed definite 

 flexure, inasmuch as the white slit-images separated. With this tube of 

 hard rubber a value of the order of = 6Xio 9 was obtained. With the offset 

 (vised throughout the following work) the apparent modulus increased to 

 E = pX io 9 and the white slit-images remained in contact. Thus far horizon- 

 tal spectrum fringes had been the criterion of measurement. It was found 

 just as easy and more accurate to use the achromatic fringes. 



In three series with the same tube the values came out as 



io~ 9 =7.8, 7.8, 8.1 



All these data are apparently much too low. Consequently the 

 hard-rubber tubes a were provided with thick brass caps b, as 

 shown in figure 31, the small conical projection fitting a re- 

 entrant cone in the beam F. Nevertheless the value now found 

 ( = 6Xio 9 ) was even lower. There is thus no doubt that the 

 section of the tube is not uniformly strained in a short solid, 

 whereas in a long slender body the stresses are soon equalized. 



>3 



Part of the sectional area of the short solid may in fact be quite 

 free from strain. Hence the device shown in figure 3 2 was next 

 tested, where a relatively thin rod of hard rubber r is surrounded by brass 

 caps b and c with closed ends. The caps fit the rod loosely and room is left 

 between them for compression. For hard-rubber rods of the dimensions 

 L=i.7 cm., A =0.2 4 cm. 2 , and in three series of experiments the moduli 

 io 9 E = 6.g, 7.5, 7.5 were computed, showing therefore no marked change 

 from the preceding data, in spite of the greatly diminished sections. In 

 the next experiments shaped rods like figure 33 were used directly. For 

 the dimensions L = 2.2 cm., A =0.41 cm. 2 , the moduli were found to be 



EXio 9 = 9.6, 9.6, 8.4, 8.4 

 in four series made after different periods of loading. 



