138 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



A resilient, straight bronze wire, 21=22 cm. long, was selected for the purpose, 

 0.24 mm. in diameter and weighing 0.0044 gram per centimeter. This was 

 trussed (fig. 175), at about one-fifth of the length from the end of the needle 

 by a thinner brass wire, stretched from the top of the hanger and mirror sup- 

 port. As the truss was not sufficiently stiff, laterally, to support the two shots, 

 (m = 0.6295 gram each), it had to be strengthened by a thin glass stem 0.47 mm. 

 in diameter and 14 cm. long. This was tied on at its ends with fine silk thread 



and a little wax. Hence the stem would offer no appreciable purchase to the 

 radiant forces except (unavoidably) at the shot, which were about 4.5 mm. in 

 diameter. The results showed that the limit had already been reached in the 

 preceding needle, so far as the stem is concerned. 

 The constants* of the apparatus were thus 



M = 949 grams. m = 0.6295 gram. ^ = 4.3 cm. /=n.ocm. L = 26icm. 



Tested by a small sphere (ball-bearing, diameter 0.633 c" 1 -. mass 1-0415 

 grams) of moment of inertia TV = 0.041 73, the slightly modified quartz torsion- 

 fiber showed a period of T = 5.06 seconds. This, in the equation, 101, makes 



7 = io~ 7 Xi-735 Ay Ay = 0. 3 84 cm. 



The same result was obtained with the needle itself vibrating in vacua. 

 The moment of inertia was here TV = 152.4, to which 5.2 per cent were to be 

 added to allow for the accessories (wire truss, hanger, and mirror) . The period 

 found was T = 3i3 seconds. This in the equation gives 



7 = io~ 7 Xi.726 Ay, or Ay = 0.385 cm. 



In fact, it seems to me that with care as to the increments of N contributed by 

 the parts of needle, this method is preferable, in spite of the tediousness of 

 finding the long period. 



*R and / were found by measurement with a distant telescope and a scale near Mm. 



