150 Dr. W. F. G. Swann on the Magnetic Field 'produced 



in the furnace experiments. 96 grams of the powder were so 

 enclosed. After more than a month the amount of emanation 

 collected in the tube was investigated by exhausting an 

 electroscope, after first carefully rating it, and filling it with 

 air drawn slowly through the U-tube. The gain in the rate 

 of discharge was less than 3 scale-divisions per hour ; barely 

 more than 2-J. This represents an amount of radium no more 

 than 1*5 xlG -12 grm. The emanation in the 96 grms is 

 about 211 x 10 ~ 12 . Hence the radium content of this powder, 

 as determined by the emanation contained in it, would be 

 underestimated by less than one per cent. The error is, of 

 course, quite negligible. 



The method of extracting radium emanation by fusion is 

 applicable to other investigations than rocks and minerals. 

 Natural waters, such as river waters, &c, would certainly be 

 best dealt with by this method; the residues from large 

 quantilies, obtained by evaporation, being fused with car- 

 bonates in the furuace. For such delicate work small-sized 

 furnaces are easily constructed. In the case of sea waters 

 its use seems particularly desirable. Discrepancies arising 

 from conditions of ebullition, acidification, presence of organic 

 matter, or from whatever source, must assuredly disappear 

 when the dry salts are treated in the furnace. In this case 

 the bulk of the residue left by evaporation may be much 

 reduced by evaporation of the chloride of sodium in an open 

 crucible, and, I believe, without risk of loss of radium. 



I hope to be able shortly to give results obtained by these 

 applications of the fusion method. 



I desire to thank Mr. W. Tatlow T for much kind assistance 

 in arranging for the supply of current required in the rather 

 extensive series of preliminary experiments which I found it 

 necessary to make: also Mr. L. B. Smyth for valuable help 

 in carrying out the experiments. 



X. The Magnetic Field produced, by a Charged Condenser 

 moving through Space. By W. F. G. Syvann, D.Sc, 

 A.R.C.S., Assistant Lecturer in FJiysics at the University 

 I of Sheffield *, 



Introduction. 



A SYSTEM of charged bodies moving through space with 

 velocity v, should, on the assumption of a quiescent 

 ?ether, give rise to a magnetic field which at each point is 

 equal to 47rvP, where P is the electric polarization at that 



* Communicated by the Author. Read at the Meeting of the British 

 Association, 1910. 



