produced by the Point Discharge. 473 



of the charges carried by them. The disc thus has a film of ions 

 held in the dust deposit on its surface so that, if it be placed in 

 the testing vessel and raised to the potential employed iu the 

 experiments, the electric field produced will act upon the ions 

 thus trapped and increase their energy, thus enabling them either 

 to go out from the disc and into the electrode facing it, thereby 

 increasing the charge in the electrometer, or else to go into the 

 metal of the disc, and so show no external effects. When the disc 

 is at the positive terminal during the discharge from the points 

 it will attract negative ions to itself, so that, in this case, some 

 of these negative ions should get trapped by the dust, to be 

 liberated when the disc is placed in the testing cylinder and raised 

 to a negative potential, while, on the other hand, when the disc is 

 at the negative terminal of the machine, it will attract to itself 

 positive ions, and consequently will, when placed in the testing 

 cylinder, leak positive electricity. 



In conclusion, I desire to express my deep appreciation of 

 the many helpful suggestions given during the progress of this 

 investigation by Prof. J. J. Thomson, F.R.S. 



