

Electric Current on the Photo- Electric Effect. 919 



This was a matter of some difficulty, because of the large 

 number of factors that had to be kept constant over a long- 

 time. The plate was illuminated by the light from a quartz- 

 mercury vapour lamp, the intensity ol which is very sensitive 

 to changes in the current caused by other work in the 

 building. A few long sets of readings were obtained, how- 

 ever, and were all similar. A typical curve is shown in 

 fig. 2. This occupied sixteen hours. 



To get these results the plate was first thoroughly fatigued 

 by exposure to the light for about twenty-four hours. It wa& 

 then found to give a very constant photo-electric current. 

 Then a small current was parsed through the plate, and 

 readings taken at intervals, until it was determined that no 

 change in the photo-electric current was occurring. In this 

 way by successive increases in plate current, a current was 

 reached where a small increase in the photo-electric current 

 occurred. Beyond this point the increases became greater 

 and greater up to the large current, where the photo-electric 

 current after reaching a maximum began to decrease. 

 Beyond this point the photo-electric current could be 

 further increased by a large rise in the plate current, but 

 consistency vanished at this point (2'2 amperes in fig. 2). 



With the current off, the plate rapidly fatigued again, 

 usually going to a point below the original value. If, now, 

 an attempt was made again to go through the process just 

 described, the results were very erratic. The increase 

 started at a slightly lower value of plate current and rose 

 very irregularly. For some increases of plate current there 

 was an abnormally large rise followed by constancy for 

 several further increases, followed further by another 

 abnormal increase. The final value reached in the first run ■ 

 with a given plate reached in some cases 100 per cent- 

 increase. 



Sputtered Plates of Bismuth. 



Because of the fact that bismuth behaves very differently 

 in the form of a sputtered film (especially the failure to 

 change resistance in a magnetic field), several sputtered 

 plates of different thicknesses v\ ere tried. They all gave 

 results similar to the thick plates, except that they could be 

 run right up to the point at which the films burned out, 

 without any inconsistency appearing. A curve for such a 

 plate is attached (fig. 3). 



A zinc plate was also tried and was found to give similar 

 results, but of smaller magnitude. 



There seem to be only two ways to account for the effect 



