K M. Wellisch — Mobilities of Ions in Air. 587 



In all the experiments described in this paper the ions were 

 drawn to the gauze by a potential of 20 volts applied to A and 

 the case of the vessel ; this potential is denoted by a and is of 

 course of the same sign as that of the charge carried by the ion 

 whose mobility is under consideration. 



Diagram of Connections and Experimental Method. — The 

 complete diagram of connections is exhibited in fig. 3, and 



Fig. 3. 



needs little explanation. As is shown in this diagram two 

 commutating discs were employed ; one of these (2) which was 

 9 cm in diameter had 20 fiber segments whereas the circumfer- 

 ence of the other (1) (6-J cm in diameter) was half fiber and half 

 metal. Four brushes were used on commutator 2 in order to 

 ensure good contact ; the collector ring had two brushes and 

 commutator 1 of course only one. The motor was worked 

 generally on 110 volts which afforded approximately 42 revo- 

 lutions per second. The speed counter was attached perma- 

 nently to the system and the time for 5000 revolutions was 

 observed several times during: the course of a series of read- 



