592 Mr. G. C. Simpson on 



air was oivino- up a necrative charo;e. It also ao-rees with 

 Villari^s experiments ; but in order to see this the arran^^e- 

 ment o£ his apparatus must be considered. 



Ill order to obtain a good insulation Villari phiced the 

 metal to be examined in the end of a glass tube 30 cms. long. 

 Sometimes this metal consisted of a few filings or pieces of 

 wire, at others the whole end of the tube was filled with 

 tightly packed filings or strips of metal. He then found 

 that when only a little metal was in the end it became 

 negatively charged, while the tightly packed metal obtained 

 a positive charge. A connexion between friction and charge 

 appeared obvious. But in reality the friction itself had 

 directly nothing to do with the phenomenon. The reason 

 for the different charges being that as Villari worked icith a 

 constant pressure driving tlie air tlwough the tube, the air took 

 longer to reach the metal when there was much friction to 

 overcome than it did when there was little. In the latter 

 case the air reached the metal during the time it was giving 

 up a negative charge ; in the former this state had been 

 passed, and a positive charging of the metal was the conee- 

 quence. That this is the real explanation could be easily 

 shown with my arrangement of the experiment, for on 

 placing a plug of cotton-wool in the far end of the tube and 

 keeping the same pressure in the box as before, the first 

 metal tube which had always before become negatively 

 charged became positively charged. That is, a positive or 

 negative charge could be given to the metal tube by simply 

 increasing or decreasing the time taken for the ionized air to 

 reach it quite independently of the friction with which the 

 air passed over the metal itself; in fact, the positive charge 

 in my experiment was obtained when the air passed over 

 the metal with the smaller friction, this being exactly con- 

 trary to Villari^s result. 



This brings Zeleny's, Townsend's, and Villari's results into 

 line, for the process which leads to the charging may be con- 

 sidered as follows : — Imagine a small part of the stream of 

 air separated from the rest, and follow it in its course along 

 the tube. When it first enters, the negative ions, on account 

 of their greater mobility, move quicker than the positive ions 

 into the neighbourhood of the walls where they are absorbed. 

 As this leaves a greater vokime-density of positive than of 

 negative ions behind, a state will soon be reached at which 

 the greater number of positive ions balances the greater 

 velocity of diffusion of the negative. Up to the point in the 

 tube where this takes place the air has been giving up a 

 negative charge to all bodies over which it streams ; at the 



