688 



brass tube by two small pieces of ambroid; round about this we 

 have once more a brass cylinder as mentioned above. 



At the ends of the rods terminals are affixed in order to fasten 

 the required connecting wires to the storage battery ; also the screw 

 of the torsion head, the bottom plate / and that of the ionisation 

 cylinder ƒ possess such screws, in order to bring the upper needle 

 a, the measuring space c, and the ionisation cylinder ƒ to the 

 potential desired. 



The arrangement of the electrometer having been explained in 

 this elal)orate way, some particulars should now be added in relation 

 to some special purpose for which the apparatus has to be employed. 

 If, namely, one wishes to use it for measuring the radio-activity of 

 emanations, the ionisation cylinder must be exhausted, and therefore 

 it must be possible to close it hermetically. Without taking particular 

 precautions this cylinder would communicate by various ways with the 

 air outside; among others along the axle and the walls of the ambroid 

 cylinder: to prevent this, the needle in the lower ambroid cylinder 

 has been cemented air-tight, while between this and the upper rim 

 of the lower cylinder of the guardring a ring-shaped cavity has 

 been filled with piceine (cf. Fig. 3). Thereupon, in order to prevent 

 leakage along the lower rim of tlie guardring and then along the 

 screws or to the centre, a rubber ring was inserted (in Fig. 3 the 

 first ring mentioned from the centre) which fits closely in a ring- 

 shaped groove in the ebonite, cut a little outside screws i, and 

 prevents the air to enter. In the same way a second rubber ring- 

 on the inside of screws 2 (cf. Fig. 3) prevents leakage from the 

 cylinder along these screws or to the outer rim. At the bottom the 

 guardring _^, besides having a wide outer rim, still possesses a iiarrow 

 rim turning inside, to prevent the ambroid, in consequence of 

 difference in pressure of air, from being pressed inward. Further 

 the closure of the cylinder at the top (by the lid K) is brought 

 about in the same way as at the bottom (by the brass bottom plate) 

 viz : by means of rubber rings. 



In the wall of the ionisation cylinder are two hermetically closing taps 

 of glass for the filling or exhaustion of air or emanation. With all these 

 precautions it appeared to be possible to bring the pressure inside 

 the cylinder down to 2 mm. with the pump (Gaede's new single 

 barrel air-pump), while only after three days it was raised o<ie mm., 

 which is quite sufficient for the purpose we have in view. 



If, however, the measurement must be done with regard to solid 

 substances (direct method), which one must be able to exchange 

 quickly and in which renewal of air should be avoided as much 



