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LXXIY. Aji Experiment to Eocldlit the Loss of Negative 

 Electricitij J)y Radium. By Hon. R. J. Stiiutt, Fellow of 

 Trinity College ^ Camhridr/e*» 



SOMETIME ago M. Curie (Comptes Eendus, cxxx. p. 647) 

 showed that the /3 or cathodic rays from radium carried 

 away a negative charge from the substance, leaving a posi- 

 tive charge upon it. The experiment has not been easy to 

 repeat with the comparatively weak radium preparations 

 generally available, because a very delicate electrometer can 

 scarcely be used nnder the conditions of a lecture experiment. 

 In this note I shall describe a simple piece of apparatus 

 which exhibits the effect in a permanent and striking manner. 



A thin- walled glass tube a, hermetically 

 sealed, contains the radium preparation. 

 It is supported from above by the quartz 

 rod b. From the bottom end hang a pair 

 of gold or aluminium leaves cc. The 

 glass tube a is smeared over with a con- 

 ducting coating of phosphoric acid f, care 

 being taken that the coating is in con- 

 ducting communication with the leaves. 

 The whole system hangs from the stopper 

 d of the glass bulb /. Strips of tinfoil e e 

 at the sides of tbe bulb are connected to 

 earth. 



As long as air remains in the bulb no 

 divergence of the leaves is observed, since 

 the radium rays make this air a con- 

 ductor, and consequently neutralize any 

 difference of potential between the leaves 

 and the tinfoil strips e e. But if a good 

 vacuum be made in the vessel the leaves 

 soon begin to diverge, owing to loss of 

 negative electricity by the radium. This 

 divergence increases until the leaves touch 

 the tinfoil strips. When this happens they 

 are discharged and collapse. The cycle 

 then recommences. 



In my apparatus the radium used was 

 about ^ gramme of a very weak prepara- 

 tion, one of those first on the market. ^ 



* Communicated by the Author. 



t This is a very convenient method of making a glass surface conduct- 

 ing for electi'ostatic experiments. The phosphoric acid can be trusted 

 not to dr}'- up under the desiccating action of phosphoric anhydride. 



