24 Mr Zeleny, On Convection Currents and on the [Oct. 31, 



U is a gold-leaf suspended midway between the plates and is 

 connected to a pole of the battery E to the other pole of which 

 the two plates are joined. 



The motions of the gold-leaf are observed by means of a 

 microscope. 



When everything is arranged symmetrically and the rays have 

 access to the whole space between A and B there is little or 

 no motion of the leaf when the rays are turned on. 



If by means of the lead plate L the rays are cut off from the 

 space between the leaf and one of the plates, say A, then the gold- 

 leaf is deflected towards the side traversed by the rays, that is 

 towards B. 



The attraction is therefore greater on the side where conduction 

 is going on than where it is not. 



When the plate B is removed altogether, the leaf is deflected 

 towards A by the existing electrostatic attraction. 



Allowing the rays to pass between the plate and the leaf 

 increases this deflection so that, in a particular case with a distance 

 of 1*5 centimetres between the two, the passage of the rays 

 increased the deflection by an amount equal to that produced by 

 an addition of ten volts to the existing potential difference of 

 eighty volts. 



This does not indicate the total increase in the potential 

 gradient at the plate, because the leaf is also acted upon by a 

 counteracting pressure due to the gas motion described in § I., 

 but as it is difficult to estimate the value of this pressure all that 

 the above experiment enables us to say is that the gradient at the 

 leaf when the rays were acting was more than | of the gradient 

 existing previously. 



It is possible to arrange the above apparatus so that an air 

 current will assist the deflection of the gold-leaf. 



When both plates are used, if instead of being kept at the 

 same potential one, say B, is connected to the leaf, then if the 

 rays are passed between the leaf and this plate, the leaf is deflected 

 towards the plate although this is at the same potential. 



Two effects combine to produce this result. 



Because of the small width of the leaf some lines of force near 

 the sides of the box extend from A to B, and others starting from 

 A reach the leaf U on the side turned towards B. 



As these latter are in the path of the rays there are some ions 

 reaching the leaf from and producing a deflection towards B. 



It was also found that the convection currents in the gas at 

 the sides of the box were from B to A, but in the centre between 

 A and U there was a return current directed from A towards U 

 which on reaching the leaf went around both edges of it, thus 

 tending to move the leaf towards B. 



