the Radiations from Radioactive Substances. 9 



quadrants. As most of the experiments were carried out 

 during the very dry Canadian winter, it was very essential to 

 screen the electrometer and connexions with testing appa- 

 ratus by wire gauze. Unless precautions of this kind were 

 taken, every movement of the observer produced sufficient 

 frictional electrification to disturb the electrometer. For the 

 same reason and also for convenience the quadrants were 

 separated by a cord connected to a suitable key and operated 

 at a distance. 



The method of observing the rate of leak was as follows: — 

 A seconds-pendulum was placed before the observer. At 

 the instant of passing the middle point of its swing the 

 quadrants were separated by a sudden pull of the cord. 

 After ten or more swings the connexion between testing- 

 apparatus and the electrometer was broken by means of an 

 insulated key, operated by a second cord. The deflexion 

 of the electrometer-needle when it came to rest was then 

 observed. 



The number of scale-divisions passed over, divided by the 

 time between the separation of the two keys, was taken as a 

 measure of the rate of leak. This method is more accurate 

 than the usual one of observing the time the electrometer- 

 needle takes to pass over say 100 divisions of the scale. The 

 final deflexion is independent of the amount of damping and 

 of any oscillation or irregularity in the movement of the 

 electrometer-needle. 



In experiments with uranium, thorium, and polonium a 

 very thin layer of the material was employed. This is 

 essential in the case of thorium oxide, in order that the 

 rate of leak due to the emanation from it may be negligible 

 compared with the rate of leak produced by the ordinary 

 radiation. In dealing with radium a very small amount 

 of material was dusted by means of a gauze as uniformly as 

 possible over a platinum plate. For the specimen of radium 

 employed the rate of leak due to the emanation and rays 

 deviable by a magnet was in this way rendered negligible 

 compared with the rate of leak due to nondeviable radiatiom 

 Suitable capacity was, if necessary, placed in parallel with 

 the electrometer to reduce the rate of leak. 



In figs. 3 and 4 (p. 10) curves are given for the absorption of 

 the different radiations by thin aluminium foil and Dutch-metal 

 respectively. In order to plot the curves on the same scale 

 the rate of leak for the bare radioactive plate is in each case 

 taken as 100. The average thickness of aluminium-foil was 

 •00036 cm., and of Dutch-metal '00012 cm. The curves arc 

 given for two specimens of radium, marked G and E, which 



