from air and ethyl bromide. 103 



upwards through a second aluminium window d into the secondary 

 ionization chamber B. A portion of the primary rays passed out 

 through a third aluminium window e, into an ionization chamber 

 P where its intensity could be measured. 



The secondary ionization chamber B was of a different form to 

 that employed in the previous experiments. It is very difficult 

 to obtain the " saturation current " across a cylindrical ionization 

 chamber, with a central wire electrode, such as was previously 

 employed. An elementary calculation will shew that the field 

 near the outer cylinder is only a small fraction of the average 

 intensity between the electrodes, and so, to obtain saturation in 

 this part of the gas it is necessary to apply a potential between 

 the electrodes much greater than would be required in the case 

 of a uniform field. As ethyl bromide is a somewhat difficult gas 

 to saturate, the cylindrical form of ionization chamber was 

 abandoned in favour of the form shewn in the figure. The 

 central electrode is here a thin sheet of aluminium leaf stretched 

 on a circular frame of copper wire. It is insulated by a quartz 

 tube passing through an earthed guard ring ; the latter in turn, 

 being insulated from the case (which is charged to a potential of 

 480 volts) by an ebonite stopper. The ends f, f of the chamber 

 are of aluminium foil which is held in position by a similar device 

 to that employed for the windows in the box A, and described in 

 a previous paper. The various joints are made air-tight by means 

 of sealing wax. 



The chamber B is so placed with respect to A, that no secondary 

 radiation from the walls of the latter can enter it. The currents 

 through the two chambers B and P are measured by Wilson elec- 

 troscopes in the usual way. 



The ionization chamber B is filled with ethyl bromide vapour 

 at some suitable known pressure (about 200 mm. of mercury). 

 The ratio of the currents through B and P is then measured with 

 the box A filled first with air and then with ethyl bromide vapour 

 at a known pressure. From the ratio of these two quantities we 

 can find the relative amounts of ionization produced in the ioniza- 

 tion chamber B by the secondary rays from air and ethyl bromide. 

 Corrections have to be applied for the absorption of the secondary 

 rays from the gas by the aluminium windows d and/"; and in the 

 case of ethyl bromide, for the absorption of the primary and 

 secondary rays in the gas of the gas chamber A. Finally we have 

 to correct for the fact that the air and ethyl bromide are employed 

 at different pressures. The various data required are known, and 

 have been given in a previous paper on the passage of Rontgen 

 rays through gases*, and the various corrections required can be 

 easily performed. In this way it was found that the amounts of 

 * Proc. Roy. Soc. Vol. lxxxii. p. 103 (1909). 



VOL. XV. PT. II. 8 



