if Common Lead could be separated into Isotopes. ii7. 9 > 



both its atomic weight and density are found to be inter- 

 mediate between those of its two isotopes, and that therefore 

 an appropriate mixture of the two isotopes would have the 

 same mean atomic weight and density as ordinary lead. If 

 this idea of the real nature of lead were correct, it would seem 

 to be possible that some separation of its two constituents, 

 which would differ by about 1 per cent, in density, might 

 be effected by centrifuging the lead while in the liquid 

 state. Such a separation could be most easily detected 

 by determinations of the density of the lead from the top 

 and bottom of the centrifuging tube, and this was the 

 method adopted in these experiments. 



The centrifuge used was one constructed by Leune of 

 Paris, which runs at about 9000 revolutions per minute. 

 The lead was contained in steel tubes, which were fitted 

 with steel lids to a^void oxidation of the lead as far as 

 possible. Quartz containing tubes were first tried, but 

 were found too weak to stand the strain of centrifuging. 

 The steel tube containing the lead was heated by means of 

 a coil of asbestos-insulated nichrome wire wound round it. 

 This coil was kept in place by two collars turned on the 

 steel tube, one at each end, so that in effect it was really 

 bobbin-shaped externally. The whole tube with its heating 

 coil fitted into the usual outer metallic holder of the 

 centrifuge, which hung from trunnions in the ordinary way. 



The heating current was supplied to the coil in the 

 following manner : — One end of the heating coil was con- 

 nected to the outer metallic holder of the centrifuge which 

 made contact with the main rotating spindle of the centri- 

 fuge through the supporting trunnions of the tube. As 

 it was not desirable to pass a current through the bearings 

 of the centrifuge, a copper gauze brush was used to make 

 contact with the vertical spindle of the centrifuge, and this 

 brush was connected to one pole of the source of current. 

 The other end of the heating coil was connected by a flexible 

 connexion to an insulated horizontal copper disk which was 

 fixed on the top of the vertical spindle of the centrifuge. 

 A vertical carbon rod was used to make contact with this 

 disk. This rod was insulated from the main body of the 

 centrifuge and held in contact with the revolving copper 

 disk by a simple adjustable spring device. It was connected 

 to the other pole of the source of current, and thus the 

 circuit through the heating coil was completed. As the 

 carbon was arranged to be concentric with the axis oi 

 rotation of the centrifuge, the minimum amount of power 

 was wasted by the brush. 



