Mass of the Ions in the Electric Wind in Air. 407 



It follows that we must look to the dragging of the ions 

 through the gas from point to plate for the source of most of 

 the pressure measured on the plate ; a conclusion which is 

 consistent, so far as it goes, with the theory sketched above, 

 and indeed suggested that theory. 



Equations (iv.) and (v.) require that dP/dz should be con- 

 stant. On the assumption that the lines of equal pressure are 

 circles on the plate, centred on the point of maximum pressure, 

 it is possible to obtain the values of P by integration from 

 curves I. This was done, and the resulting values of P 

 plotted with z are shown by black dots in curves III. for 

 fifteen different values of z ; the plate D being used, and the 

 conditions being those of curves I. If we omit the two points 



Curves III. 



(P) 



DYNE 



3 



































+ / 



\ 

















'/- 

























. 











































































S\ 





~>'' 



\ 



• 













S+ 





s* 







































,' 















k y 





,' 





















a' 

















% ..* 





















** 

















S* 



°v 





















„* 



















> .^< 





















i 



















O 0-2 0-4- 06 0-8 10 12 1-4- 16 Cffl.(Z) 



for the highest value of z (see below) the constancy of dP/dz 

 is roughly indicated. The two curves also pass very nearly 

 through the origin so that it (equation (v.)) is small. It 

 appears indeed to be of negative sign for the lower curve ; 

 a possible explanation of this is considered further on. 



In these experiments the strength of the field was practically 

 uniform everywhere except close to the point : its value in 

 volts per centimetre being about 4760 for + and 3730 for — 

 discharge. This was shown by plotting the differences of 

 potential between plate and point with z ; the result being 



