Mr M c Glelland, On the Conductivity of Gases, etc. 241 



On the Conductivity of Gases from an Arc and from In- 

 candescent Metals. By J. A. M c Clelland, M.A., Cavendish 

 Laboratory. 



[Received 11 December 1899.] 



The first part of the following paper contains an account of 

 experiments on the conductivit}^ of gas through which an arc 

 discharge has passed ; the second part refers to somewhat similar 

 experiments on gas taken from the neighbourhood of an incan- 

 descent wire. In both cases the gas has considerable conductivity, 

 which can be shown to be due to ionisation, or a production in 

 the gas of positively and negatively charged carriers of electricity. 

 Both methods of producing ionisation are very suitable for a 

 study of the nature of these carriers, as we can easily arrange to 

 have a plentiful supply, and this supply can be maintained fairly 

 constant. 



Experiments have been made with the arc and with the in- 

 candescent wire in air, oxygen, and carbonic acid gas ; the relation 

 between current and E.M.F., the recombination of the carriers, the 

 velocity of the carriers under electric force, and other points have 

 been investigated. 



The Arc in Air. 

 Relation between current and E.M.F. 



(1) The arrangement in Fig. 1 was used to determine how 

 the current in the conducting gas depended on the potential 

 difference between the terminals ; all of the apparatus shown in 

 the figure is not required for this experiment but may be described 

 here as it shall be referred to again. 



The primary of an induction coil was connected in series with 

 an alternating supply, and the terminals of the secondary of the 

 coil were joined to the electrodes t and t'. These electrodes were 

 of brass with platinum pieces inserted in their ends, so that the 

 arc was formed between platinum terminals. The terminals were 

 fixed in corks in a glass funnel F, and one of them provided with 

 an ebonite handle so that it could be moved in to form the arc 

 and then drawn out to any required distance ; the length of arc 

 used was generally 2 or 3 mms. A brass tube A 1/7 cms. in 

 diameter was fixed in the upper end of the glass funnel, and 

 a tube D filled with glass wool, to stop dust particles, leads into 



