Ions and Electrons through Gases. 39 



deflexion: k closed: S closed on b side:/ and g both closed : 

 earth-key K open : motor and commutator running but not 

 operating on account of the short-circuit at g : capacities B 

 and C included in the system. 



(ii.) k opened : S switched to the right. 



(iii.) g opened, if it is desired to work with added capacity. 



Or 



(iii.) Capacities B and C cut out and g then opened, if it is 

 desired to work without added capacity. 



It will be seen from the foregoing that the effect of 

 induction was to alter only the reversed field whose value did 

 not need to be known at all accurately. The electrometer- 

 needle always experienced a small kick when the switch g 

 was opened, but this quickly subsided and the current was 

 measured with the needle in steady motion, the midpoint of 

 the range of deflexions being so chosen as to coincide with 

 the zero of the instrument. 



4. Experimental Results. 

 (A) Electrons in Gases. 



In fig. 4 of the previous paper typical curves were given 

 showing the relation between the current due either to 

 positive or negative ions and the potential (x) for various 

 pressures ; from such curves the critical potential V could 

 be deduced and the ionic mobility determined. In figs. 5 

 and 7 of the same paper there were given the curves corre- 

 sponding to the negative carriers in air at relatively low 

 pressures ; the characteristic feature of these curves is their 

 compound nature resulting from the independent passage 

 through the gas of electrons and ions. It is convenient to 

 designate as I curves the former type which is due solely to 

 ions, while the latter type may be referred to as EI curves ; 

 moreover, those curves or parts of curves which arise solely 

 from the motion of electrons will be called E curves. 



On resuming the experiment, an investigation was made of 

 the gases C0 2 and H 2 . The C0 2 was prepared in a Kipp's 

 apparatus by means of the action of dilute HC1 on marble 

 and was passed through NaHC0 3 Aq. in order to remove acid 

 fumes : the H 2 was obtained by the action of dilute HC1 on 

 zinc, and was passed through KOH Aq. In each case the gas 

 was passed through a series of tubes of CaCl 2 and P 2 5 in 

 order to remove traces of moisture. A series of I and EI 

 curves was obtained for these gases under various conditions, 



