McClelland and Kennedy — The Large Ions in the Atmosphere. 75 



connected tube B. Along the axis of A was placed a metal rod C, of 2 mms. 

 radius, which was joined to one terminal of a number of small storage-cells. 

 The tube D led to the fan used to draw the air through tube A and the air- 

 meter to measure the flow. The tube B projects beyond the tube A so that 

 the stream of air enters A with considerable velocity ; and the rod Cis not 

 carried to the end of A. This arrangement ensures all the ions of one sign 

 being collected on A ; they are not prevented from entering by an electric 

 field, as the tube A during an observation is never much different from zero 

 potential. 



Sufficient voltage was applied to G to remove all the small ions ; the 

 number of large ions collected was negligible or the necessary correction could 

 be made. 



The arrangement was such that the electrometer could readily be joined 

 to the apparatus for measuring the small ions, or to that for measuring the 

 large ions. 



Mobility of the Ions. 



The first point investigated was the mobility of the large ions ; and some 

 care was taken to decide whether the large ions were all of the same mobility, 

 or whether they were a mixture of ions of different masses. 



The method consisted in plotting the saturation curve carefully, and the 

 theory of the method is well known. In the larger apparatus, suppose 

 the velocity of the air-stream at a distance ij from the insulated plate joined 

 to the electrometer is u, and that the uniform potential gradient is V volts 

 per cm., then the transverse velocity of an ion is kV, where k is the mobility 

 of the ion. Hence, if x is measured along the middle plate from the point 

 where the air leaves the electric field, and if the outer plates are charged 

 positively, we have, for the path of a positive ion the equation, 



cly__kV 



dx 11 ^ ' 



If a be the width of the tube, then 



ha V dx = a u dy. 



Hence, for saturation of the ions of mobility k, we require a voltage V 



given by the equation 



kaVl = a ju dy, (2) 



where / is the length of the insulated central plate. 



But the integral a Judy taken over the section between the central and 

 one of the charged plates is the quantity Q of air passing through this cross- 

 section per second. 



