. Radio-activity . ntact difference of potential between the plates, and would beproduced whether the rays were charged or not. The ionization ofthe gas is greatly diminished by placing over the active materiala metal screen which absorbs the a rays, but allows the /3 rays topass through with little absorption. The rapid loss of any charge communicated to the top platecan be very much reduced, either by diminishing the pressureof the gas surrounding it or by enclosing the plate with suitableinsulators. In their experiments to determine the amount ofcharge carried by the radium rays, M. and Mme
. Radio-activity . ntact difference of potential between the plates, and would beproduced whether the rays were charged or not. The ionization ofthe gas is greatly diminished by placing over the active materiala metal screen which absorbs the a rays, but allows the /3 rays topass through with little absorption. The rapid loss of any charge communicated to the top platecan be very much reduced, either by diminishing the pressureof the gas surrounding it or by enclosing the plate with suitableinsulators. In their experiments to determine the amount ofcharge carried by the radium rays, M. and Mme Curie* usedthe second method. A metal disc MM (Fig. 26) is connected with an electrometerby the wire T. The disc and wire are completely surrounded byinsulating matter ii. The whole is surrounded by a metal envelopeEEEE connected with earth. On the lower side of the disc, theinsulator and the metallic covering are very thin. This side isexposed to the rays of the radium R placed in a depression ina lead plate A Electrometer Fig. 26. The rays of the radium pass through the metal cover andinsulator with little absorption, but they are completely absorbedby the disc MM. It was observed that the disc received a negativecharge which increased uniformly with the time, showing that therays carry with them a negative charge. The current observedwas very small. With an active preparation of radium f, forming * M. and Mme Curie, C. R. 130, p. 647, 1900. + The activity of the radium preparation was not stated in the paper. 122 NATURE OF THE RADIATIONS [CH. a layer 2*5 sq. cms. in area and 2 mms. thick, a current of the orderof 10~ amperes was observed after the rays had traversed a layerof aluminium 01 mm. thick and a layer of ebonite 3 mm. current was the same with discs of lead, copper, and zinc, andalso when the ebonite was replaced by paraffin. Curie also observed in another experiment of a similar characterthat the radium itself acquired a positive charge. This necessa
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