SECTIONAL TRANSACTIONS.— A. 325 



the streams of air are not running continuously, and thus the desiccating 

 agent is conserved for long periods. A strip of gold-beater's skin in the 

 chamber itself (which is provided with a fan for stirring the air) carries 

 electrical contacts which complete circuits when the length of the strip 

 deviates in either direction from the desired length. The circuit which is 

 completed when the gold-beater's skin becomes too short, operates a fan 

 driving a stream of moist air into the chamber ; similarly the other circuit 

 drives dried air into the chamber. 



Afternoon. 

 Visit to Stonyhurst College and Observatory. 



Monday, September 14. 



Discussion on The production and technical applications of high voltages 

 (10.0). 



Dr. T. E. Allibone. — The production and application of high voltages. 



The paper reviews the many types of generators at present in use in 

 engineering and physical laboratories for the production of high alternating, 

 direct and impulse voltages, with special reference to the purpose for which 

 each generator is constructed. 



In electrical engineering such generators are used for the testing of insula- 

 tion, and details of the A.C. and impulse voltage tests on insulating material 

 and on assembled apparatus are given. In the physical laboratory such 

 generators are used for the most part to produce swift moving electrically 

 charged particles such as electrons or positive ions for the generation of 

 X-rays or neutrons or for investigations on atomic structure. Details of 

 the apparatus most commonly used for these purposes are given. 



Dr. G. W. C. Kaye, O.B.E., and Mr. W. Binks.— The ionisation 

 measurement of short-wave radiation. 



The success and safety of cancer treatment by X-rays and radium, 

 whether singly or combined, largely depend upon the accurate measurement 

 of the quantity or ' dose ' of radiation administered. The most satisfactory 

 method available is a physical one involving the determination of the 

 ionisation produced in air by the electrons liberated by the radiation. 

 For X-radiation the air-ionisation unit of quantity called the rontgen has 

 been accepted internationally, but the difficulties hitherto encountered in 

 measuring very short-wave radiation, such as the y-rays from radium, have 

 given rise to doubts concerning the feasibility of expressing X-ray and 

 radium measurements in a common unit, a procedure admittedly desirable. 

 The difficulties of realising the rontgen in the case of y-rays are due to the 

 long ranges of the electrons liberated in air (up to 10 or 12 ft.). The authors 

 show that these difficulties can be overcome by the use of a very large 

 parallel-plate ionisation chamber, of effective dimensions about 12 ft. by 

 10 ft. By this means the unification of X-ray and radium dosage measure- 

 ments has been effected. Such a chamber, while serving its purpose as 

 an ultimate standard of reference, is of course quite impracticable for medical 

 purposes. The present results, however, establish the fact that for every- 

 day practice small ionisation chambers having appropriately thick walls of 

 ' air-equivalent ' material — a type already in use for X-ray measurements — 

 can be successfully employed for y-rays. 



