PHYSICAL MEASUREMENT OF X-RAYS. — BRONSON 21 



my disposal of estimating the potential on the tubs. In 

 the various experiments the currents used varied from 5 

 to 0.3 milliamperes and the spark gap from 8 to 18cm. 



Experimental Methods and Results. 

 The present investigation may ba divided into three 

 di'stinct parts: 



(1) To discover whether the action of Roantgen 

 rays on a photographic plate is proportional to the 

 ionization in the air near the plate, and thus to ba able 

 to determine the proper length of time for an exposure; 



(2) To compare the action of the rays on a Sabour- 

 aud pastille with their ionizing action; 



(3) To find an accurate method of comparing the 

 hardness or penetrating power of the raj's. 



(1) 



In order to test the first point the ionization current was 

 measured with some object, for example several sheets of 

 glass, aluminum or lead, covering the opening F of tha 

 testing vessel. Then a photographic plate, covered by th3 

 same object, was exposed to the action of the rays, produced 

 under the same conditions as before. The X-ray plates 

 used were either Ilford or Wellington, the developer was 

 carefully prepared according to the maker's directions, and 

 great pa'.ns was taken to alwa3^5 use the same quantity of 

 fresh developer at the same temperature and to develop for 

 the same length of time. 



It was discovered by preliminary experiments that an 

 exposure of 5 sec. gave a satisfactory negative, when the 

 deflection of the galvanometer was 40mm. with a bundle 

 of rays 100 sq. cm. in area. Thereafter the time of exposure 

 of any plate was adjusted to be inversely proportional to th3 

 current through the galvanometer, that is, the product of the 



