ft 



32i C. Bams — Thomson's Constant. 



Art. XXXY. — Thomson's Constant, <?, Found in Terms of 

 the Decay Constant of Tons. %oithin the Fog Chamber ; by 

 Carl Barus. 



1. Introductory. — In the last paper,* an account is given of 

 certain tentative experiments to determine Thomson's elec- 

 tron, by aid of the fog chamber and a separate well -leaded 

 cylindrical electrical condenser. The results obtained for e 

 agreed well with the accepted values. It was shown that the 

 constants of coronas are determinable from purely optical 

 consideration of diffraction and interference, and that the 

 accuracy of the method may be enhanced by using the mer- 

 cury 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 

 gratuitous. The results seen in the fog chamber are a com- 

 plication of the effects of primary and secondary radiations 

 together with a very marked exhaustion displacement of the 

 iocs. The maximum ionization 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 in the same apparatus. 

 This is the case in the experiments of the present paper. 



2. Electrical condenser— fog chamber. — It is therefore 

 necessary to make the fog chamber itself an electrical condenser, 

 and this is easily done if the chamber is cylindrical, by install- 

 ing a tubular core of aluminum closed in the inside of the 

 chamber and running axially from end to end. 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 easily) is the outer sur- 

 face 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. (1) 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 



*C. Barns: This Jour., xxvi, pp. 87-90, 1908. 



