330 Mr. E. B. Rosa on the Determination ofv, the 



The results exhibited in the preceding Table have been 

 divided into four groups. The first group consists of 

 seventeen values found before the condenser was taken apart 

 to measure its electrostatic capacity. During this time the 

 upper half of the shell was lifted, and the ball adjusted two 

 or three times. The values found by the fast fork are more 

 uniform than the others, and average somewhat lower. The 

 second group extends from May 4 to 9 inclusive, when the 

 condenser had been set up again. There were two small glass 

 tubes, about 5 mm. in diameter (and drawn out considerably 

 smaller where they projected through the shell one to two 

 centimetres into the space within), which had once been used 

 to pass charging-wires through. The wires had been with- 

 drawn, and it was supposed that the glass tubes had no 

 appreciable effect. The holes were together only -5 oV(J °f ^ ne 

 area of the shell, and the tendency of the glass to slightly 

 increase the capacity would tend to counterbalance the 

 decreasing effect of the holes. When the condenser was 

 set up the second time, the tubes were intentionally left 

 out and the values of Group II. were noticed to be larger 

 than those of Group I. No cause could be discovered for 

 this increase (which indicates a less electromagnetic capacity), 

 but the tubes were replaced and Group III. taken. The mean 

 of this group is as large as that of the preceding group. The 

 tubes were now again withdrawn, and the holes covered with 

 gold foil, making the inner surface of the shell continuous. 

 Group IV. gave values averaging almost exactly the same as 

 Group I. The circumstances were alike in other respects so 

 far as is known, the usual variation in the circumstances of 

 the observations, as already explained, occurring in all the 

 groups. I do not think the presence or absence of the tubes 

 could affect the capacity appreciably ; they were altogether 

 too small, probably not filling over a thirty-thousandth of the 

 space between the ball and the shell. But that there was a 

 difference in the actual capacity of the condenser when 

 Groups I, and IV. were taken from its value when II. 

 and III. were obtained seems almost certain. As yet I have 

 not become satisfied as to the cause of this difference ; but it 

 seems probable that, in putting the condenser together, some 

 obstruction lodged between the two halves of the shell and 

 prevented them from coming completely together. Had they 

 been separated a few hundredths of a millimetre only, the dif- 

 ference in question would be fully accounted for. The surfaces 

 of contact are very accurately ground and polished, and 

 loosening the screws does not cause them to separate, as 

 proved by the capacity remaining constant. That the low 



