Danysz and Duane — Electric Charges of a- and (S-Rays. 297 



zontal, and, therefore, normal to the axis of the cylinder. The 

 magnetic field deflects the rays, and a study of the geometry of 

 the figure shows that none of the rays passing through C are 

 stopped by the diaphragm D, if the radii of curvature of their 

 paths are greater than 10 cm : and that no ray for which the 

 radius of curvature is less than 3'8 cin can pass through C. The 

 magnetic field strength was measured by means of a standard- 

 ized fluxmeter, and attained a maximum value in the experi- 

 ments of about 8000 gauss. 



All the rays passing through the hole C enter the metal 

 cylindrical box E connected to a piezo-electric quartz and an 

 electrometer, by means of which their charges are measured. 

 The top of the box E is made thick so as to stop the greater 

 portion of the 7-rays, which, if they came out at the top might 

 produce secondary rays in sufficient quantity to alter perceptibly 

 the charge of electricity measured. The bottom of the box 

 consists of a thin sheet of aluminium; its distance above the 

 cylinder B being about 8 mm . 



To protect the wires and measuring instruments from the 

 penetrating 7-rays the quartz and electrometer were placed in 

 such a position that the rays had to pass through the long pole 

 piece of the electromagnet before reaching them, and all wire 

 connections were imbedded in paratfine or ebonite, which were 

 covered in turn with metal tubes put to earth. 



The piezo-electric quartz had been calibrated some years 

 before by means of a standard condenser and standard cadmium 

 cells. It was recalibrated by measuring the same ionization 

 current once by the quartz and again by a galvanometer, the 

 figure of merit of which had been determined by means of 

 standard resistances and standard cells. A weight of one kilo- 

 gram suspended from the quartz produced a displacement 

 of 5*86 electrostatic units of electricity. 



The two cylindrical boxes B and E were supported inside a 

 larger metal box F, in which a high vacuum was produced by 

 means of a Gaede mercury pump. The cylinder B (as well as 

 E) was insulated from the box F so that it could be charged to 

 any desired potential, while the box F remained always put to 

 earth. 



A series of measurements were made with the hole C 

 covered with a sheet of aluminium *004 mm thick, and with the 

 cylinder B raised to different potentials. The curves in figure 

 2 represent the results. It will be noticed that both positive 

 and negative currents reach maximum values for electric fields 

 of a few volts, the negative potential required to produce satu- 

 ration being a trifle larger than the positive. 

 - These currents are not due to ordinary ionization in the 

 residual gas, for ionization currents at such low pressures 



