276 Forces at Surface of a Needle-Point discharging in Air. 



the error in/ due to this cause must therefore have been less 

 than 1 per cent. 



As to the electrical effect o£ the N ions, it may be suffi- 

 ciently described by saying that when they enter the ionizing 

 layer they behave like B ions, and before entering it they play 

 the part of the fixed plate. 



And since their mechanical effect on the pull is so small 

 it follows that under the conditions of current and size of 

 point considered above the conclusions already arrived at as 

 to the connexion between f and P for single discharging 

 points of either sign will still hold when N ions are supplied. 



Reaction of the Electric Wind. 



It may not be out of place here to refer to the assumption 

 sometimes met with in text-books and elsewhere, that the 

 reaction of the electric wind is to be found at the discharging 

 point. 



Reaction there must of course be — somewhere — when the 

 wind is started, and its amount must be that of the momentum 

 given per second to the ions ; but only an extremely small 

 part of it is to be found at the point itself. 



It is true that if a needle with a sharp point and its other 

 end blunt be electrified until the point discharges, it will tend 

 to recede from its discharging end. The electric windmill is 

 a well-known instance of this. But the needle moves because 

 it it pulled more strongly at the blunt than at the sharp end, 

 not because it is pushed back at the latter. If the blunt 

 end be electrically shielded the needle tends to come forward, 

 and to about the same extent that it did before discharge 

 set in ; it is the shield which now exhibits reaction by its 

 increased tendency to move backwards. 



But the effect on the shield is only part of the wind reaction. 

 When discharge starts the distribution of electricity on all 

 the surrounding conductors changes, and the electrical forces 

 on them alter in such a way that the resultant of these 

 alterations acts in the opposite direction to the wind, and is 

 equal to its reaction. 



The wind reaction is thus to be found upon the electrified 

 portions of both electrodes ; but the portion which probably 

 feels it as little as any is that part of the point surface at 

 which the discharge actually occurs. 



