in producing Electric Conductivity in Gases. 305 



produced at most of the collisions. The current when the wire is 

 positive increases rapidly at the higher voltages. 



The experiments at this pressure and at lower pressures would 

 indicate that a smaller voltage is sufficient to produce ionisation 

 by the collisions of the negative ion than what is required to give 

 the necessary velocity to the positive. But they show that the 

 positive ion we are dealing with can be given the velocity necessary 

 to produce ionisation. 



7. In experimenting with the ionisation produced by an 

 incandescent wire in a gas at low pressure, there are many in- 

 dications that we are dealing not only with the ions produced 

 from the molecules of the gas but that we are actually getting 

 ions off from the hot wire itself. 



When the gas is at atmospheric pressure the number of 

 positive ions is in excess of the number of negative except when 

 the temperature of the wire is very high, when the amount of 

 positive and negative is approximately the same. When the 

 pressure is reduced the negative ions are greatly in excess, even at 

 moderately high temperatures. Using the apparatus described, 

 the current when the wire is negative may be 50 times what it is 

 when the wire is positive, the pressure being 1 mm. or less. Such 

 a difference from what is observed at atmospheric pressure sug- 

 gests that at low pressure we get a copious supply of negative 

 ions from the wire itself. 



Again, the current when the wire is negative varies little with 

 the pressure while the pressure is very small ; for example, the 

 current was practically constant when the pressure was varied 

 from gL mm. to ^io mm - This suggests that at such pressures 

 the ions resulting from the ionisation of the gas molecules were 

 small in number compared with those coming from the wire. 



Possibly the positive ions which are active in producing 

 secondary ionisation also come from the wire, which may account 

 for the apparent difference between them and the positive ions 

 investigated in other cases of ionisation. 



However, more experiments are required before entering into 

 any further discussion of this nature. The wire may produce 

 ionisation at considerable distance from itself, in which case even 

 when the wire is positive, the negative ions would travel for some 

 distance through the gas and in this way produce secondary 

 ionisation, but this action could scarcely explain the effects 

 obtained. 



It is intended to investigate more completely some of the 

 effects described in the paper. I desire to thank Prof. Thomson 

 for kind advice during the course of these experiments, which were 

 carried out at the Cavendish Laboratory some time ago. 



