THE TRANSFORMATIONS OF ELEMENTS 529 



electroscope, it will attract ions with unlike charge in sufficient 

 number to discharge itself (since the charge on the electroscope 

 is likewise due to the presence of free electrons, if negative, or 

 to free positive ions). 



It is found that ordinary air always contains a certain very 

 small number of ions, and the natural leak of an electroscope 

 is due to their presence. Again, the production of ions is 

 strictly proportional to the number of rays — e.g. if two quantities 

 of uranium oxide are taken, one twice the weight of the other, 

 and they are spread out in thin films of the same thickness, 

 so that the first has twice the exposed area of the second, it will 

 emit twice as many rays as the second ; they will produce twice 

 the number of ions, having twice the effect on the leaves of an 

 electroscope ; hence it will be discharged in half the time. 

 Similarly if equal quantities of two radioactive materials are 

 taken, and consecutively introduced inside an electroscope, and 

 it is found that the rate of discharge of the first is twice as fast as 

 that of the second, then it is said to possess twice the activity of 

 the second. The measure of the discharge-time gives a measure 

 of the radioactivity. By comparing the effect produced by a 

 substance from day to day, a measure of the variation in its 

 radioactivity is obtained. 



The most convenient instrument for these measurements, 

 however, is some form of quadrant electrometer, preferably the 

 modification invented by Dolezalek in 1901. This has a very 

 light needle of silvered paper, carrying a small mirror, and 

 suspended by a very fine quartz fibre between the four quadrants. 

 One pair is connected to earth, the other to an insulated metallic 

 plate, facing a second which carries the radioactive matter to be 

 tested. Through action similar to that in the case of an electro- 

 scope an electric stress is set up between the two pairs. The 

 needle, previously charged to a very high potential, is repelled 

 from one pair of electrodes towards the other ; on account 

 of its extreme lightness it moves at a uniform rate. The 

 time taken to pass two fixed points is observed by means of 

 a stop-watch. As in the case of an electroscope, this time is 

 inversely proportional, other conditions being the same, to the 

 degree of radioactivity of the deposit under observation. The 

 natural leak of the electrometer is deducted from all other time 

 measurements. The electrometer is standardised from time to 

 time against a definite amount of uranium oxide, which is known 



