. Radio-activity . he glass vessel was ex-hausted, and the charge communicatedto P by the ^ rays was measured byan electrometer. In a good vacuum, the magnitudeof the current observed is a measureof the number of ^ particles absorbedby the upper plate*. The followingtable shows the results obtained whendifferent thicknesses of tin foil wereplaced over the radium. The second table gives the ratio y where /„ is the rate of discharge observed before theFig. 31. absorbing screen is introduced. The mean value of the absorption constant X. was deduced from the equation y = e~^^ where d is the thickn
. Radio-activity . he glass vessel was ex-hausted, and the charge communicatedto P by the ^ rays was measured byan electrometer. In a good vacuum, the magnitudeof the current observed is a measureof the number of ^ particles absorbedby the upper plate*. The followingtable shows the results obtained whendifferent thicknesses of tin foil wereplaced over the radium. The second table gives the ratio y where /„ is the rate of discharge observed before theFig. 31. absorbing screen is introduced. The mean value of the absorption constant X. was deduced from the equation y = e~^^ where d is the thickness of matter traversed. The values included in the brackets have not the same accuracyas the others. There is thus a wide difference in penetratingpower of the /3 particles emitted from radium, and some of themare very readily absorbed. * It is presumed that the results were corrected, if necessary, for the dischargingaction due to the ionized gas, although no direct mention of this is made in thepaper by IV] NATURE OK THE RADIATIONS 139 When a lead scrtnai 8 lums. tlvick was i)lacc(l over the radium—a thickness sutticiciit to absorb all the readily deflectable /3 rays—a small negative charge was still given to the plate, corre-sponding to 29 per cent, of the maximum. This is a very muchsmaller value than v^as observed by Paschen (see Fig. 30). This Thickness ofTin in rams. X 0^00834 •869 175 0-0166 •802 132^5 0-0421 -653 101-5 0-0818 -466 93-5 0-124 ■359 82^5 0-166 •289 74-9 0-205 •230 71-5 0-270 •170 65-4 0-518 •065 ^ 53I 0-789 •031 1 44 1 1-585 -0059 1 32 I 2-16 •0043) 25 j difference may, in part, be due to the fact that, in Paschensexperiments, a large proportion of the slow velocity electrons wereabsorbed in the glass tube of ^5 mm. thickness containing theradium. Seitz also determined the relative thickness, compared withtin, of different substances which reduced the negative chargecommunicated to P by a definite amount. A few of the numb
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