124 W. Duane — Effect of a Magnetic Field 



vice versa) simply by reversing the key I. In making measure- 

 ments one notes the positions of the image of the gold leaf on 

 the scale of the reading telescope, corresponding to small 

 equal positive and negative potentials ; then, breaking the 

 contact at G, one measures the time required for the image 

 to pass from one of these positions to the other. The total 

 change in the potential of the gold leaf divided by the time is, 

 of course, practically proportional to and a measure of the 

 current. On closing the key G and verifying the positions 

 of the image on the scale one can immediately detect, and 

 correct for, any change in the zero or the sensitiveness of the 

 instrument. 



In order to be sure that the current observed is due to the 

 ionization of the gas under investigation and not to foreign 

 gases or secondary rays or other disturbing causes, I adopted 

 the method of filling the ionization chamber represented in 

 the figure. In the first place the gases in the ionization cham- 

 ber should be pumped out and kept pumped out long enough 

 to remove the greater part of the gases occluded in the metal 

 surfaces. On beginning an experiment the stopcock " a " is 

 closed and the system of tubes including the mercury pump 

 is filled through drying tubes containing H 2 S0 4 and P 2 6 with 

 . the gas to be investigated. With the stopcock " b " closed this 

 gas is then pumped out, and the process repeated several times. 

 When, during the final pumping, the pressure has decreased 

 to several millimeters of mercury, the stopcock "c" is closed, 

 thus leaving a small quantity of gas in the reservoir " b c," and 

 after this the best vacuum obtainable with the mercury pump is 

 produced in the rest of the apparatus including the ionization 

 chamber and the reservoir " d," which contains carbon, the 

 gases in the carbon having been removed by previous prolonged 

 heating to a high temperature in a vacuum. Finally, the mer- 

 cury pump being cut off by the stopcock " e," the pressure is 

 reduced still further by cooling the reservoir " d " with liquid 

 air. If one measures the ionization current during the time 

 the carbon is absorbing the residual gas, one finds that this 

 current gradually decreases, sometimes changing sign, and 

 after about an hour reaches a value that can be detected, but 

 is negligible compared with the currents to be measured. 

 When this condition has been reached, the reservoir a d " is 

 cut oif by closing its stopcock, and the stopcocks first u c " and 

 then " f " are opened and closed several times, admitting each 

 time to the ionization chamber the small quantity of gas 

 between them. This process is repeated until the ionization 

 currents become sufficiently large to be measured, and the 

 required pressure obtained. By this method one can be sure 

 that the currents are really due to the ionization of the small 



