60 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



cult to interpret this excess by supposing that the method is inadequate ; for 

 the effect of gravitational attraction between the disks, which has been 

 ignored, would be a virtual increase of P . Since, on the average, pressure 

 excess, ppo, is very small as compared with p in the actual case, equation 

 (n), seeing that p <2 p 2 o = 2p (ppo) becomes 



and 



7 R 



which is identical in interpretation with equation (2) above, where po = < 

 therefore leads to the same conclusion. 



33. Observations, continued. Presence and absence of electrical contact. 



Notwithstanding the improbability of electrical effects, it was thought neces- 

 sary to test the case directly. Accordingly, in table 4 and fig. 36, series i 

 to 8, experiments are recorded with the plates not in metallic contact, series 

 i to 5, and with the plates in metallic contact, series 6 to 8, respectively. The 

 behavior in both cases is virtually the same, when the shift of zero is taken 

 into account. Observations are plotted in time series, with the last observa- 

 tion marked by a circle, and they are in each case continued until the motion 

 of the plate is retrograde, whereupon the real oscillation of the plate begins. 

 To throw further light on the subject, a Leclanche' cell was introduced in 

 series 9 and 10 and removed in series 1 1. The lighting circuit of the room was 

 placed more remote in series 12 and the system earthed in series 13 and 14 

 for both fixed disks. 



10 

 08 

 46 

 44 

 M 

 00 



5 6 



ft 



f- 



3 



70 



70 



70 170 



W 



J?0 



S15 



r 



! 



IB 





VMS ZOO 



*/5 



40 60 80 100 



140 



160 m m 22.0 m 



FIG. 36. 



That the differences observed are most probably referable to the flexure of 

 the pier in shifting the zero, is shown in series 15 to 1 8, where the observations 

 are made on one side only, with the distance between disks gradually increas- 

 ing, as the fixed plate moves from position 2.15 to position 2.00, i.e., 0.15 cm. 

 The interval of observation was 48 minutes ; but the interval and final reading 



