n 



Mobility of the Negative Ion. 



445 



cut out of its centre a circular hole 3 cm. in diam. opposite 

 to the strip o£ platinum and covered with metal gauze gh. 

 The gauge employed is shown in figs. 1 and 2. A vane 

 abed is cut out in the form shown in fig. 2 from a sheet of 

 aluminium 0*1 mm. thick, and is suspended by a fine brass 

 ribbon z attached to a torsion head X which may slide on a 

 rod v projecting from the plate B. On its lower part the 

 Fie:. 2. 



vane is provided with a mirror r, a small weight g, and an 

 arrangement for damping. The earthed cage Jc forms an 

 electrostatic protection for the vane. The apparatus is 

 placed on a brass plate pierced by insulated electrodes and 

 is covered with a bell-jar provided with a window of plane 

 glass, so that the image formed by the mirror may be 

 undistorted. Before being admitted into the bell-jar the 



