36 Mr. J. A. McClelland on the Conductivity of 



The two curves agree approximately, which would show 

 that the rate of recombination of the ions is proportional to 

 the square of the number present in the dissociated gas. We 

 should expect the conductivity to fall off in this way, as the 

 number of collisions between positive and negative carriers 

 will be proportional to the square of the number present, and 

 each collision means a definite loss of conductivity. The 

 curve a must diverge considerably from the curve b nearer to 

 the flame, since b if continued back would give an infinite 

 conductivity before the flame is reached. The rate of recom- 

 bination closer to the flame would not seem to be as rapid as 

 it should be if it continued to be proportional to the square 

 of the number of ions. 



In the experiment from which the above curve is drawn 

 the absolute velocit}^ of the current of gas up the tube was 

 not determined, so that the time is only expressed in arbitrary 

 units. In another experiment, however, where a wider tube 

 was being used, and the velocity of the current up it was 

 measured by a wind-gauge and found to be 130 cm. per 

 second, the conductivity fell to half its value between two 

 points 17 cm. and 27 cm. distant from the flame. The con- 

 ductivity thus fell to half its value in ^ of a second. Of 

 course the rate of loss of conductivity rapidly diminishes with 

 the conductivity, so that for electrometer determinations it is 

 quite convenient to work with the gas more than 1 second 

 after it has been drawn away from the flame. 



3. Velocity of Carriers under an Electromotive Force. 



We can easily determine the velocity with which the 

 positively and negatively charged ions travel under an electric 

 force. The apparatus shown in fig. 5 was used. 



A is a cylindrical tube *85 cm. radius, with two terminals 

 B and placed one above the other in the axis of the tube — 

 each 6*5 cm. long and *2 cm. radius. These terminals are 

 insulated and supported by ebonite plugs. To prevent the 

 insulation of the ebonite being spoiled by moisture depositing 

 on its surface, it is convenient to shape the ebonite as shown 

 in the figure, so that the insulating surface is not exposed 

 directly to the current of the gas. 



A piece of tubing T leads from A to an ordinary water- 

 pump P connected to the water-mains. The pump is con- 

 nected by a glass tube as shown with a large glass vessel V, 

 which is fitted with the two tubes a and b — a just passing 

 through the cork, and b going to the bottom of V. The 

 water and air from the pump enter by c, and the tap in a 

 can easily be set so that the vessel remains about half full of 



