EMISSION AND TRANSMISSION OF RONTGEN RAYS. 151 



The point of support of the gold leaf should not penetrate more than a few 

 millimetres within the case of the instrument. A useful length of leaf is 3'5 cms. ; 

 the end of the leaf should just swing clear of the charged plate when the instrument 

 is tilted on end with the charged plate downwards. 



It should then be found that with the base of the electroscope horizontal, and the 

 charged plate at a potential in the neighbourhood of 200 volts, the sensitive region is 

 near the middle of the window provided. 



If the charged-plate end of the electroscope be tilted too high, and the potential on 

 the plate too large, the leaf will be unstable over a part of its range, i.e., it will not 

 return to its zero when earthed. If considerable sensitiveness is aimed at, it is not a 

 bad plan to first get the leaf in the unstable condition ; then by means of the adjusting 

 screws gradually lower the plate end of the electroscope, and at the same time 

 diminish the potential on the plate until the leaf is stable and gives a region with the 

 required sensitiveness. The greater the sensitiveness, the more limited the region of 

 that sensitiveness an extra volt on the charged plate, or a fraction of a turn of the 

 tilting screws, may cause, a large alteration in the sensitiveness. 



With a short leaf, a large potential (240 volts or. more) on the plate and an 

 excessive tilt (the charged-plate end very high) will be necessary. A longer leaf 

 takes a smaller potential (120 volts or so), and may require a considerable reverse tilt, 

 i.e., with the charged-plate end lowest. 



If, when the potential of the leaf is altered, it creeps very slowly and uncertainly to 

 its final position, it usually means bad electrical contact between the leaf and its 

 support. 



