132 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



filamentary wire; i. e., the balls at the end of the needle offer an appreciable 

 surface in any case. 



What militates seriously against such a straightforward explanation, how- 

 ever, is the enormous effect of the presence of air. In a partial vacuum i to 10 

 cm. the error will not exceed 20 per cent in the 

 summer (constant temperature), or 30 per cent 

 in the winter (steam-heated room) . In a plenum 

 of air, the results obtained may be 5 or 10 times 

 too large, depending on the lateral area of the 

 needle. It is difficult to escape the inference, 

 therefore, that the presence of the ball B pro- 1/ e 

 duces air-currents within the apparatus between 

 ball and needle, and that it does this by modify- 

 ing the radiation on its own side as compared 

 with the radiation on the other side of the needle. 

 The effect of a ball B, colder or hotter than the 

 effective temperature on the other three sides, 

 might thus be the same and its presence would be accompanied by a spurious 

 attraction in relation to gravitation. If the ball B were to tranquilize the 

 medium between m and B, there would be repulsion, and this practically 

 never supervenes, so far as the present experiments go. 



101. Experiments with slender needles. To carry out the suggestions of 

 the preceding paragraph, three types of apparatus were used. In the first 

 of these (IV), the quartz fiber of the above apparatus (No. 7) which had shown 

 deflections from A^ = 2.6 to 7.2 cm. in its old location, was inserted. The case, 

 the needle, and the position, however, were new. The case, specially made for 

 experiments in vacua, was of the form shown in cross-section (normal to the 

 needle) in figure 165. The rectangular body BB was of cast brass with an 

 internal clear space 1.2 cm. wide, 8 cm. high, and 25 cm. long. Long, thick, 

 rectangular rubber gaskets r, r were placed on the inner rim, on which the glass 

 plates P, P' reposed. The channel between the plate and the body was filled 

 with resinous cement cc, poured in when molten and the brass body warm. 

 The joint so made proved to be perfectly tight, though it was a little difficult to 

 remove the plates for the change of needles, etc. 



The needle N was supported by the quartz fiber q, surrounded by a glass 

 tube, and the joint made by an annular cup C, into which melted cement was 

 poured for sealing, as indicated in the figure. A similar cup-like contrivance 

 sealed the torsion-head at the top of the tube. To turn the head, the wax was 

 temporarily melted. 



To mount the two shot (0.581 gram each and about 0.45 mm. in diameter) a 

 phosphor-bronze wire, 22 cm. long, 0.64 mm. in diameter, and weighing 0.649 

 gram, was found to be adequate. The finished needle, with mirror and hanger, 

 weighed 1.931 grams. The stem weighing but 0.0296 gram per centimeter, 

 an allowance could be made for the gravitational attractions actuating it. 



