70 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



Table 8, however, admits of a number of preliminary estimates of the 

 decrease of repulsion (/) with d, the distance apart of plates, for we may write, 



/= 65.2X2*= 130* dynes, nearly. 



These values are given in column 12 of table 8 and in fig. 416, with the ex- 

 ception of the first, which is liable to be anomalous from actual contact. The 

 second observation also seems to be in error for some reason not detected. 

 The others make a compatible series. The forces found in the above work 

 (paragraph 10) lay between 1.3 and 0.4 dynes, for distances of the same order 

 of value, but which were not quite the same in the two positions of the fixed 

 disks. If we take the results in table 6, which are probably the best, A7V = 

 (0.049 0.014)72=0.018 cm.; d= (0.031+0. 097)72=0.064 cm.; 7=65. 2X 

 0.018 = 1.17 dynes, the results of/ and d, as shown by the cross in fig. 4iB, 

 fit in very well with the present data obtained from electric attraction. The 

 repulsion therefore has throughout been found of the same order of magnitude. 

 The pressure corresponding to the above thrust / is found (as above) by 

 dividing by the area A of the disks, whence 



We may then compute the attraction of the disks per gram of air film, at 

 a distance h from the disk, similarly to the ordinary case of the barometric 

 formula, 



or - 



Kr an 



Thus, if one can detect the variation of / with h, the molecular attraction of 

 the disk per gram of air should be discernible. 



37. Conclusion. By the application of displacement interferometry to the 

 deviations of the horizontal pendulum, I find that two parallel rigid plates 

 whose distance apart is of the order of i mm. and less repel each other, in air, 

 with a force far in excess of their gravitational attraction. This force in- 

 creases rapidly (certainly as fast as the inverse square) as the distance of the 

 plates decreases, and vice versa, but can be recognized beyond a millimeter of 

 distance. For brass plates 20 cm. in diameter and i mm. apart, the repulsion 

 in question is of the order of 0.5 dyne and therefore equivalent to a pressure 

 of 0.0015 dyne-cm, or roughly io~ 9 atmosphere. It is in excess of any electric 

 repulsion due to the absolute voltaic potential of the disks. The suspended 

 plate reaches its position of equilibrium gradually, the motion progressing at 

 a retarded rate through infinite time, in a way characteristic of the viscosity 

 of the film of air between the plates. 



I have estimated the intensity of the force both from the repulsions of a 

 vertical plate suspended from the horizontal pendulum on opposite sides of a 

 fixed parallel identical plate ; also by charging pairs of plates to a given differ- 

 ence of potential for a given distance apart. So far as can be seen, the repul- 

 sion is caused by the condensation of air on the surface of the plates by molec- 



