PREFACE. V 



free fog chamber, where the nucleation (ionization) was found from the 

 constants of the coronas obtained upon exhaustion, or vice versa. 



Using the method which depends essentially on the known velocity of 

 the ions in the unit electric field and my earlier values of the constants 

 of coronas, a few rough tests of the charge of the electron gave consistent 

 values. There was, however, an inherent difficulty of great importance, 

 the nature of which has already been referred to the ionization differs in 

 different parts of the fog chamber and the extreme ratios may exceed 2 

 to i. It does not follow, therefore, that the mean ionization observed 

 in the fog chamber is the same as that obtaining within the heavy leaded 

 electrical condenser. To secure this identity the fog chamber itself must 

 be the condenser. 



The method was, therefore, varied by using the cylindrical fog chamber 

 (glass wet within, put to earth) with its axial core of charged aluminum 

 tube both as an electrical condenser for the measurement of current and 

 as a fog chamber for the measurement of ionization. The end of the 

 aluminum tube within the fog chamber is hermetically sealed ; the other 

 is open without for the introduction of the sealed tubelets containing 

 radium. By properly adjusting these along the axis an approximately 

 uniform ionization within the fog chamber is obtainable. The trials made 

 seemed promising enough to make it worth while to repeat the determi- 

 nation of e by Thomson's method, using, however, the mercury lamp as 

 a source of light and a purely optical method for the measurement of the 

 nucleation as suggested above. Results will be given in a later report. 



The correlative method of determining e in terms of the decay con- 

 stant of the ionization has also been tried. If A r be the number of ions 

 in the fog chamber due to the radium in the aluminum tube when the 

 latter is not charged and n the number when it is charged the constant e 

 ma be written 



where C is the capacity and v the volume of the cylindrical condenser 

 fog chamber and V the (constant) fall of potential of the core per second, 

 due to the current passing through the ionized air within the chamber. 

 In this case very large potentials (250 volts) may be used, and a graduated 

 Exner electroscope suffices for the measurement of V. 



The experiments by the decay method in the second part of Chapter 

 IV, made for the present merely to test the standardization of the fog 

 chamber, as detailed in the earlier publications of the Carnegie Insti- 

 tution of Washington, nevertheless lead to very acceptable values of 

 e, even at the enormous ionizations (exceeding 500,000 nuclei per 

 cubic centimeter) employed. 



The values for e in Chapter IV were too high, even with the admission 

 that negative ions only are caught in the fog chamber employed. The 



