30 



Mr. J. A. McClelland on the Conductivity of 



in the flame itself but in the gases taken from the flame has 

 been investigated : here the conditions are simpler and the 

 nature of the electrodes used is not so important. For 

 papers on this part of the subject see Giese, Wied. Ann. xvii. 

 1882, xxxviii. 1889. 



The method used in the experiments is simple ; the gas is 

 drawn along a brass tube inside which is placed an insulated 

 terminal which can be raised to any required potential, and 

 the conductivity of the gas passing this terminal is measured 

 by the rate of fall of potential of the terminal as given by an 

 electrometer. For experiments on conductivity inside a 

 flame a sensitive galvanometer may be used, but when one is 

 dealing with the conducting gas drawn from the flame the 

 conductivity is too small to permit the use of a galvanometer. 

 The electrometer method enables us to work with the gas at 

 distances from the flame convenient for the experiments. 



1. The Relation between the Current and the 

 Electromotive Force. 



To determine the manner in which the current through 

 the conducting gas depends on the E.M.F., the arrangement 

 in fig. 1 was used. A is a metal tube 3*7 cm. in diameter 



Fig. I- 



EARTH 



FLAME 



with a funnel attached to it. Beneath this funnel an ordinary 

 rose bunsen-burner is placed and regulated to give a steady 

 flame. The products of combustion pass up the tube A with 

 a velocity constant at each point as soon as the tube has 

 attained a steady temperature, and the bunsen-burner is 

 large enough to ensure that the stream of conducting gas is 

 as nearly as possible the same at all points in a horizontal 

 section of the tube. B is a brass rod 10 cm. long and '5 cm. 

 diameter, placed in the axis of A and insulated by an ebonite 

 plug C. It is connected to one pair of quadrants of an 

 electrometer E. A is connected to earth, and the two pairs 

 of quadrants are first connected together and to one pole of a 

 battery of storage-cells, the other pole of which is to earth ; 



