proceedings: philosophical society 



253 



The Bureau of Standards purchased in the open market twelve 

 pieces of lead glass for X-ray protective purposes. No information 

 could be obtained about them other than that "the protection is 

 adequate." These pieces averaged about 5 mm. thick. The average 

 protective coefficient was 10 per cent, i.e., on the average each piece of 

 glass was equivalent to 0.5 n m. lead. Two pieces (the nost expensive 

 of the lot) had each a protective coefficient of 0.9 percent and so were 

 worthless for X-ray protection. They were replaced by the vendor. 



Samples of lead rubber ."howed an average protective coefficient of 

 22 per cent. One piece went as high as 32 per cen . 



Correspondence was carried on with several firms relative to the 

 improvements to be desired in protective materials. The results have 

 been most gratifying. It is now possible to obtain materials with the 

 following properties : 



SUBSTANCE 



Plate glass . . 



Rubber 



Bowl (tube holder) 



PROTECTIVE 



COEFFICIENT 



per cent 



18 

 48 

 16 



lead content 

 (by weight) 



per cent 



50 

 94 



47 



equivalent lead 

 for particular 



SAMPLE 



1.16 

 1.17 



f minimum) 



The density of practically^ every sample of protective material 

 received at the Bureau of Standards has been determined. It has been 

 shown that there is a linear (approximately) relation between the density 

 and the protective coefficient. 



Discussion: The paper was discussed by Messrs. Silsbee, Sosman, 

 Briggs, Humphreys, and Webster. 



Mr. P. T. Weeks then presented a paper on The efficiency of production 

 of X-rays. 



The term efficiency of production of X-rays is ordinarily taken to 

 mean the ratio of the total energy of the X-rays emitted either from 

 the tube or from the target to the energy supplied to the tube. The 

 value of this efficiency has been found by actual measurement to be of 

 the order of one tenth of one per cent. It has been found to be nearly 

 proportional to the atomic weight of the target. A simple theory of 

 the method of production of X-rays would indicate that the efficiency 

 should be proportional to the potential applied to the tube, and experi- 

 ments have verified this conclusion so far as it is applied to the general 

 X-radiation. The manner in which the energy of the characteristic 

 radiation varies with the voltage has been only partly determined. 



In general two effects have been used for measuring X-ray energy, 

 the ionization produced in a gas and the heatjng effect on absorption. 

 In most of the determinations of the efficiency only a small energ^^ input 

 could be used and widely varying results were obtained b}^ different 

 observers. The author used a Coolidge tube with much larger energj- 

 input and determined the value by a bolometer method. His results 



