Magnetic State of Iron and its Photo-Electric Properties. 343 



field perpendicular to the surface. The source of the photo- 

 electrous was the face of the core of a small electromagnet. 

 This core was insulated from its exciting coil by a glass 

 sleeve, and raised to a high negative potential by connecting 

 it to the negative pole of a high-voltage battery. A few 

 millimetres in front of the iron surface a copper gauze grid 

 was placed. The grid was supported on a sulphur pillar 

 insulating rod, and in area was considerably larger than the 

 end of the iron core. A lead was taken from the grid to a 

 Wilson-Kaye Tilted Leaf Electrometer, by which instru- 

 ment the current across the air-gap could be easily measured. 

 To illuminate the iron a beam of light from a small arc was 

 employed. The arc is perhaps not a very satisfactory source 

 of constant ultra-violet light, but, unfortunately, a quartz 

 mercury vapour lamp, which would have been more suitable, 

 was not available. However, by placing an ammeter in the 

 arc circuit and keeping the current constant by adjusting 

 the length of the arc a fairly constant source of light could 

 be obtained. The effect of casual variations in the bright- 

 ness of the arc is further considered in the sequel. 



The arc itself was placed inside an iron projecting lantern. 

 No lenses were used to concentrate the light on the iron 

 specimen, which was illuminated through a small quartz 

 window at one end of the lantern. The lantern serves to 

 shield the arc from draughts, and also prevents any ions 

 from the arc reaching the grid which might cause spurious 

 effects. It was found better to earth the body of the lantern, 

 which thus acted as a screen preventing any direct electro- 

 static effect of turning on the arc affecting the electrometer. 

 There is a possibility that the exciting field of the iron 

 specimen might have a direct effect on the arc itself. The 

 lantern also tends to screen the arc from this action, but, to 

 further insure against it, an iron plate about a centimetre 

 thick was placed between it and the grid. A hole in the 

 centre of this plate allowed the light from the arc to pass 

 through to the grid. 



The grid itself is screened from any direct electrostatic 

 effect due to turning on the current in the exciting coil by 

 an earthed conducting piece of brass foil. The end of the 

 coil nearest the grid was also kept permanently earthed, as 

 it was found that, before this precaution was adopted, 

 turning on the exciting current deflected the electrometer 

 leaf slightly when the latter was isolated at the time. The 

 earthing of the screen and coil completely cured this com- 

 plaint. As a matter of fact, however, the effect is of no 

 great importance, as in practice the electrometer leaf was 



