STANDARDIZATION OF FOG CHAMBER. 59 



THOMSON'S CONSTANT e, EXPRESSED IN TERMS OF THE DECAY CON- 

 STANT OF IONS, WITHIN THE FOG CHAMBER. 



48. Introductory. In the last paper* an account is given of certain 

 tentative experiments to determine Thomson's electron, by aid of the 

 fog chamber and a separate well-leaded cylindrical electrical condenser. 

 The results obtained were in keeping with the accepted values. It was 

 presumed that the constants of coronas are determinable from purely 

 optical considerations of diffraction and interference, and that the 

 accuracy of the method may be enhanced by using the mercury lamp as a 

 source of light for the coronas. There was, however, one grave misgiving, 

 inasmuch as the distribution of ionization within the fog chamber varies 

 in marked degree from place to place for any given position of a sealed 

 radium tube, and that the mean value assumed was in a measure gratui- 

 tous. The results seen in the fog chamber are a complication of the effects 

 of primary and secondary radiations, together with a very marked ex- 

 haustion displacement of the ions. The maximum ionization (Chapter 

 III) does not coincide, as a rule, with the position of the radium, and there 

 is no reason why the ionization in the fog chamber should be quite 

 identical with the ionization produced by the same radium tube in the 

 electrical condenser, unless both are one and the same apparatus. This is 

 the case in the experiments of the present paper. The results also show- 

 that the same method may be applied under the circumstances of the 

 last paper, where e was obtained in terms of the velocities of the ions. 



49. The electrical condenser fog chamber. It is therefore necessary 

 to make a fog chamber itself the electrical condenser, and this is easily 

 done, if the chamber is cylindrical, by installing a tubular core of alumi- 

 num closed in the inside of the chamber and running axially from end 

 to end, as in fig. 18. This core is charged to a definite potential and made 

 the inner surface of the condenser, while the scrupulously clean inner 

 wall of the glass chamber (to which water adheres evenly) is the outer 

 surface and put to earth. Finally, the radium, contained in small sealed 

 tubelets of aluminum, is placed within the length of the axial aluminum 

 tube or core in such a way as to make the ionization within the fog 

 chamber uniform a condition vouched for in case of the occurrence of 

 uniform coronas on exhaustion, from end to end of the chamber. 



There are thus three currents to be determined: 



(i) The conduction current due to inevitable leakage between the 

 condenser surfaces. This is made a minimum and nearly negligible in 

 value by keeping the aluminum core out of the condenser when not in 

 use and by sheathing it with an annular air-space beyond the condenser. 

 It is found experimentally by direct measurement in the absence of 

 radium. 



*C. Barus: Am. Journ. Sci., xxvr, pp. 87-90, 1908. 



