Current of Electricity from a hot Platinum Wire, etc. 499 



is incorrect. For what becomes of the ions that are deflected 

 by the magnetic field ? They are deflected in such a way 

 that their path which was originally parallel to the axis of the 

 tube now becomes perpendicular to it. The ions thus strike 

 the walls of the glass tube. The glass being very dry and the 

 pressure of the gas low, the insulation of the inner surface of the 

 tube is very good. Thus the glass will get charged up by the 

 deflected ions. When the discharge is small this will not interfere 

 much with the action of the magnet on the ions. When the 

 discharge is big, however, the glass will get charged to a high 

 potential so that there will be a strong electric field opposing the 

 deflexion of the ions. We should accordingly expect fewer ions 

 to be deflected the larger the current, i.e. the higher the tempera- 

 ture. This explanation was not arrived at until some experiments 

 had been made with a new form of apparatus which we shall now 

 briefly describe. One of the disadvantages of the first experi- 

 mental tube was that the potential difference at my disposal 

 (120 volts) did not quite saturate the discharge. This difficulty 

 was got over in the new form of apparatus by having the grating 

 of wire midway between two parallel plates. These plate 

 electrodes consisted of two brass discs 2*8 cms. in diameter and 

 were supported in a vertical plane in the glass tube A (Fig. 1) 

 by a stiff aluminium wire passing through a glass tube fitting 

 into the tube S. The discs were 8 mms. apart and were con- 

 nected together by one thin bar of brass screwed into the discs 

 at the extreme end of parallel diameters. 



^7hm^> & 



Earth 



In order to obtain as uniform an electric field as possible 

 between the wire and the discs, the former was fitted up in the 

 following way. A piece 1 cm. square was cut out at the centre of 

 a disc of platinum foil 2*7 mms. in diameter. The grating of 

 platinum wire was placed in this opening in the plane of the 

 disc. One end of the wire was welded to the disc, the other to a 



33—2 



