Radium Constants on the International Standard. 321 



■uranium was determined by Boltwood. The result was 

 expressed in terms of the 3*69 milligram standard,, which 

 has generally been referred to as the " Rutherford-Bolt- 

 wood" standard. 



A radium standard in use in the Laboratory in Manchester 

 was accurately compared for me by Professor Stefan Meyer, 

 Secretary of the International Radium Committee, in terms 

 of the Vienna standard, which has been set aside as a secon- 

 dary International standard. At the same time, Mr. Chad- 

 wick, in the Manchester Laboratory, compared by a balance 

 method the laboratory standards with a secondary standard 

 kindly lent to me by the Radium Institute of Vienna. 

 Recently Dr. Kaye has compared two of the Laboratory 

 standards with the radium standard of the National Physical 

 Laboratory. The numerous cross measurements at different 

 institutions have all been found to be in excellent agreement, 

 and bring out the reliability of the 7-ray method of mea- 

 surement. 



The original Rutherford-Boltwood standard was assumed 

 to contain 3'69 milligrams pure radium bromide. The actual 

 7-ray activity of the standard as employed in actual use was 

 found to correspond to 3'51 mg. RaBr 2 in terms of the 

 International Standard, and was thus 4*9 per cent, too low. 

 Considering the time of preparation of this standard, the 

 choice of material has turned out to be very fortunate. No 

 doubt part of the radium bromide from which the standard 

 was prepared had been converted before weighing into car- 

 bonate by exposure to the air, and this would account for the 

 high radium content, considering that no allowance was 

 made for the water of crystallization. In any case, the result 

 brings out the purity of the radium preparations sold com- 

 mercially by Dr. Giesel more than eleven years ago. 



It has been suggested to me that it would be a convenience 

 to many workers if the various radioactive magnitudes deter- 

 mined in the Laborator}' at Manchester were re-calculated in 

 terms of the International Standard. In my work * Radio- 

 active Substances and their Radiations,' most of the results 

 were expressed in terms of the laboratory standard, but the 

 heating effect, which was determined while the book was 

 passing through the press, was given in terms of the Vienna 

 Standard, which had been compared with the International 

 Standard. Confusion may consequently arise in regard 

 to the standard in which some of the results have been 

 expressed. 



Phil. Mag. S. 6. Vol. 28. No. 165. Sept. 1914. Y 



