the Radiations from Radioactive Substa?ices. 3 



order to measure with certainty the very small rate o£ leak 

 involved, a very sensitive electrometer was employed. The 

 instrument is described by Dolezalek* in a recent paper, and 

 was constructed by Herr Bartels o£ Grottingen. It was of 

 the usual quadrant type, but was provided with a very 

 light needle suspended by a fine quartz fibre. When the 

 needle was charged to 200 volts it gave a deflexion corre- 

 sponding to 1500 mins., with the telescope and scale at a 

 distance o£ about 150 cms , for 1 volt between the quadrants. 

 For the special purpose for which it was employed, it was 

 found necessary to improve the insulation of the quadrants 

 and to alter the quadrant connexions. The instrument was 

 easy to work and gave accurate results. It has been employed 

 recently by one of us f to measure the small spontaneous 

 ionization produced in the air, which has been shown by the 

 experiments of Elster and Geitel J and C. T. R. Wilson § 

 who used specially designed electroscopes for that purpose. 



In the experiments on the action of a magnetic field on 

 uranium radiation (fig. 1) a thick layer of uranium oxide was 



Fig. 1. 



&* r /> 



*fr OyM*. 



placed on the bottom of a rectangular lead box 5*7 cms. long, 

 1*8 cm. wide, and 4*0 cms. deep, which was placed between 

 the flat pole-pieces of a large electromagnet. The rays, after 

 passing out of the lead box, passed between two parallel 

 insulated plates A and B. One of these plates A was charged 

 to a P.D. of 50 volts above the earth by means of a battery. 

 The other plate B was connected to one pair of quadrants of 

 an electrometer in the usual manner. 



* Verh. d. D. Physik. Ges. iii. (1901). 



f Rutherford and Allen, Phys. Zeit. No. 11, 1902. 



% Phys. Zeit. Nov. 24, 1900. 



§ Proc. Roy. Soc. March, 1901. 



B2 



