128 PHYSICAL SCIENCE 



that they are driven towards an electrode by the 

 action of an electric force. 



The non-persistence of gaseous ions and the 

 consequent need of their perpetual renewal explains 

 the relation between current and electro-motive 

 force — a relation different from that observed in 

 liquid solutions. In solutions, as we saw, the 

 conduction conforms to Ohm's law — the current 

 is proportional to the electro-motive force. In 



Electromotive Force 

 Fig. 27. 



gases this Is not the case. For an ionizing agency 

 of constant intensity, such as a layer of oxide of 

 uranium, the current at first rises with the applied 

 electro-motive force, but soon it tends towards a 

 limit, and finally reaches a maximum, when, till 

 we approach the sparking point, no further increase 

 of electro-motive force will produce any appreci- 

 able increase of current. This saturation current, 

 as it is called, is represented by the horizontal part 

 of the curve in Fig. 27. Obviously it corresponds 



