Surface of a Needle-Pohlt discharging in Air. 275 



as well as its ends are receiving ions whereby the pressure of 

 the gas is raised at A / A / as well as at. A, and the lateral escape 

 of gas from A is consequently hindered. 



The proportionate change (reduction) in p will thus be not 

 much less than that calculated above for C ions travelling in 

 gas-proof cones, viz. 



C 0-0035 



* = 



m 



where H is 2ir and C is that part of the current carried by 

 N ions which reaches the hemispherical end of P. C is thus 

 several times less than the whole current carried to the point 

 by N ions. 



The N ions are fully formed when they reach P, so that 

 400 cm. sec. -1 is now the correct value for Y. 



In the experiments with two points, both of which were 

 discharging, a current of 15 microamperes meant of course 

 a smaller current carried by N ions, and of this a fraction 

 only arrived at the end of P. If we estimate this fraction at 

 one-fifth and calculate an upper limit for k by assuming that 

 the whole 15 microamperes v\ere carried by N ions, the result 

 is 0*016 for the largest and 0*021 for the smallest point. 



The error introduced into / by neglecting this part 

 of the momentum of the N ions is thus of the order of 

 1 per cent. 



2. The remainder of the momentum received by P from 

 the N ions. This is due to the wind set up by the whole 

 of the N ions in passing from N to the conductors connected 

 with P, instead of, as in 1, to the much smaller number which 

 reach the end of P. 



It is impossible to calculate the effect of this momentum 

 on P, but an upper limit was obtained by surrounding P with 

 a small cage of which the wires were close enough to shield 

 P electrically, but open enough to allow the wind to pass 

 freely through, the wimshurst being turned at the same rate 

 as in the actual experiments, and the pressure on P measured 

 by tilting the apparatus. Under these circumstances P 

 would be more blown against than without the cage, partly 

 because the cage would attract to itself more of the N ions 

 by reason of its size, and. partly because the current from 

 tne cage would be less than from P uncovered, and so 

 the wind from N w ? ould be less reduced by ions travelling 

 against it. 



In no case was the observed force of the wind on P greater 

 than 2 per cent, of the pull when the cage was removed, and 



