Rays of Radioactive Substances. 



317 



the electromagnet. This was covered over with a layer of 

 aluminium-foil *00034 cm. thick. 



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Special precautions were taken to completely secure the 

 electrometer and connexions from electrostatic disturbances 

 of all kinds. 



It is easy to investigate the magnetic deviation of radium- 

 rays in such an apparatus with an ordinary electrometer, as 

 the ionization action of the radium-rays is very large. 



For uranium and thorium, however, the ionization due to 

 the deviable rays is very small, and a specially sensitive 

 electrometer is required to measure the effects with accuracy. 

 The Dolezalek electrometer, described in a previous paper by 

 one of us *, was found sufficiently sensitive for this purpose. 



All the radioactive substances emit non-deviable as well as 

 deviable rays, and generally the ionizing action of the non- 

 deviable rays is much greater than that caused by the 



* PJn/s. Zeit. no. 11, p. 225 (1902). 



