352 Prof. J. J. Thomson on the Charge of 



time is nevA. If C is the capacity of the electrometer and 

 its connexions, this quantity of electricity will produce a 

 potential-difference of neuA/C between the quadrants, the 

 deflexion of the electrometer is proportional to this difference 

 of potential ; hence to deduce the value of ne from the read- 

 ings of the electrometer, we require to know the value of 0. 

 This was found by connecting with the lower plate of the 

 condenser another condenser of known capacity 0/, and again 

 measuring the difference of potential between the plates after 

 the current has been flowing for one second; this is equal to 

 imiA/(C + C) ; hence if S x , S 2 are the deflexions of the electro- 

 meter before and after C was inserted, we have 



8 1 _ C + C / 



or 



C: 



The effective capacity of the Dolezalek electrometer depends 

 mainly upon the charge on the needle, and may therefore 

 vary considerably from time to time, thus in these experi- 

 ments C ranged from 200 to 900 cms. : it is therefore very 

 necessary to determine the value of C for each observation. 

 The proceeding adopted was to take, say, six consecutive 

 readings with the condenser C (whose capacity was "001 

 microfarad) out (in three of these the upper plate was at a higher 

 potential than the lower plate; in the other three the sign of 

 the potential-difference was reversed) ; then six with it in, and 

 then another six with it out. 



The high effective capacity of the Dolezalek electrometer 

 makes it less sensitive for measuring small quantities of 

 electricity than for potential-differences. 



The value of n in the electrical experiments is the number 

 of ions per c.c. of the gas when exposed to the electric field ; 

 in the cloud experiments the value found for n is the number 

 when there is no electric field. The electric field tends to 

 drive the ions out of the gas, so that the number in the electric 

 field will be less than the other ; when the field is strong the 

 difference is very marked, but with the weak fields (about 

 J of a volt per cm.) used in these experiments the difference 

 is inappreciable; we can test whether the field is weak 

 enough to ensure this by measuring the currents under dif- 

 ferent electric fields : since u the velocity of the ion is pro- 

 portional to the electric force, if the electrometer deflexion is 

 also proportional to this force, n will be constant, I e. will 

 not alter with the force. The electric force was reduced to 



