CHAPTER XL 



GRAVITATIONAL EXPERIMENTS. 



100. Slender needles. The object in making the above experiments was 

 at the outset a mere endeavor to read the deflections of the gravitational needle 

 by interferometry. For this reason a straw shaft was used, as it facilitated the 

 adjustments. The plan succeeded without serious hardships. It was soon 

 found, however, that what was being measured was not the gravitational de- 

 flection, but a much larger value, resulting from the radiation forces simulta- 

 neously present. Unfortunately, the fiber broke in the vibration experiments 

 made to find its torsion coefficient. Using the microscope to measure the 

 diameter of the fiber and using the known rigidity, the Newtonian constant 

 came out 10*7 = 21, or about three times too large, nor were the individual 

 deflections even approximately constant. 



Thereafter, I retained the needle, as I became specially interested in this 

 interaction of radiation and gravitational forces, and a series of experiments 

 showing a very striking response to solar radiation (even when screened off 

 from the apparatus by the semi-submerged dark basement room) was carried 

 out. The results given in the preceding paragraphs point out, incidentally, 

 that if measurements of 7 are aimed at, the straw shaft is inadmissible and that 

 needles having the thinnest possible framework should be used, as offering 

 the least surface for the application of radiation pressures. 



One is tempted to interpret the discrepancies in question (at least with the 

 needle in vacuo), directly in terms of light pressure or heat-wave pressures. 

 Now, if the full solar constant be taken as 0.05 dyne per cm. 2 per second, this 

 pressure would be /=7oXio~ 6 dyne per cm. 2 and in the case of the straw 

 shaft considerably more than a square centimeter would be screened off by the 

 large lead ball outside. The corresponding gravitational forces in the above 

 experiments were about F 2.4Xio~ 6 dyne. Hence the ratio f/F is over 

 30; so that if considerably less than one-thirtieth of the solar radiation 

 penetrates the walls of the room its effect is in conflict with the gravita- 

 tional forces. 



Figure 164 will make this tentative explanation clearer. Let mm' be the 

 gravitation needle with the shot at its ends, B the large external lead ball. 

 The radiation RR, acting symmetrically at both ends of the needle, remains 

 ineffective. The radiation R'R' on the other side is screened at B, supposing 

 B to radiate less, which is almost always the apparent case in the following 

 work, though the reason for this effective screening is not quite clear to me. 

 Hence B acts radiationally as if it were an attracting body. It is not necessary 

 that R and R' be of equal intensity, but the drift of the needle (which is some- 

 times excessive) would depend on this difference. Curiously enough, these 

 forces are present in marked degree, even when the needle is a framework of 



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