Gases by the Motion of Negatively charged Ions. 199 



current through the gas does not exceed a certain maximum 

 value whatever the electromotive force may be/' 



Numerous experiments have also been made with electrodes 

 of various shapes, and it was found that the conductivity is 

 not altered when the electromotive force is reversed. 



From the experiments which are described in the present 

 paper, it will be seen that the relation between current and 

 electromotive force is not characterized by either of these 

 properties when the conductivity is produced in a gas at a 

 low pressure. There is no critical pressure or force at which 

 discontinuity occurs ; but, as the pressure is reduced, the 

 connexion between current and electromotive force gradually 

 loses the simple properties which hold at high pressures. 



At low pressures the current may be considered to pass 

 through three stages as the electromotive force is increased. 

 In the first stage, the current increases with the electromotive 

 force ; in the second stage, the current remains practically 

 constant and shows only small variations for comparatively 

 large changes in the force ; in the third stage, the current 

 rapidly increases with the electromotive force. Also, for 

 certain shapes of electrodes, a great difference can be 

 produced in the conductivity by reversing the electromotive 

 force. This is particularly noticeable when the current 

 passes between two electrodes, one of which surrounds the 

 other ; but w T hen parallel plates are used, the conductivity 

 shows only slight alterations when the direction of the 

 current is changed. In the latter case there would be no 

 alteration whatsoever if the two plates were made of the 

 same metal. We shall therefore begin by describing the 

 nature of the conductivity between two parallel plates when 

 the gas between them is at a low pressure. 



2. The apparatus used for the investigation is shown in fig. 1 

 (p. 200), and consists of two parallel plates inside an air-tight 

 vessel connected to a Topler pump. The lower plate A was 

 a thick sheet of zinc w-ith a circular window W which was 

 covered with a disc of thin aluminium. The junction between 

 the aluminium and the zinc was sealed with elastic glue, so 

 as to make the joint air-tight. 



A sheet of aluminium foil was stretched over the surface 

 of the zinc plate so that a plane face should be presented 

 to the movable brass plate B. The tube T, by which the 

 vessel was exhausted, was soldered to the back of the brass 

 plate. 



The brass plate B was 10 centimetres in diameter, and the 

 circular aperture W was 4 centimetres in diameter. 



The neck of the glass covering C was closed with an 



