738 Mr. A. E. Garrett on Electrical 



mercury. In all cases it was found that the time taken for 

 the leaf to move over one scale-division could be increased in 

 the ratio 8 : 10 by the application of a strong magnetic field , 

 showing the emission of negatively charged particles from 

 the zinc iodide to be taking place. 



When the leaf was negatively charged, no alteration in the 

 rate of leak* could be detected (for pressures as low as 15 mm, 

 of mercury) when the magnetic field was applied. 



In these experiments with the magnetic field the part 

 below the sulphur bead was shaped like L. The leaden box 

 containing the zinc iodide was made long and narrow, and 

 was placed with its long axis parallel to the lower limb 

 of the L and at a distance of 1-2 mm. from it. 



At pressures 2*54 mm. and *1 mm., when the leaves were 

 positively charged, the time taken for the leaf to move over 

 oue division with the magnet off, to the time taken with the 

 magnet on, was in the ratio 7 : 10. Practically the same 

 results were obtained for both directions of the magnetic 

 field ; therefore they were not due to any dissymmetry in 

 the apparatus. 



It was noticed on some occasions that putting on a magnetic 

 field caused a sudden discharge, such as would be caused by a 

 spark. A similar effect with a Wehnelt cathode was shown 

 by Prof. J. J. Thomson in a lecture at the Royal Institution 

 a year ago. 



E. g., when the pressure was reduced to '015 mm., and the 

 magnetic field was put on, the leaf was instantly discharged 

 if it were positively charged above a certain potential. 



When the leaf was negatively charged under the same 

 conditions, a partial discharge takes place when the magnetic 

 field is first put on, and then no further discharge takes place 

 when the magnetic field is again put on. 



There also appears to be a distinct lag when the leaf is 

 negatively charged between the putting on of the field and the 

 partial discharge of the leaf. 



In the case of both positively and negatively charged leaf, 

 there was found to be a critical potential to which the leaf 

 must be charged in order that a discharge may take place 

 when the magnetic field is put on. This critical potential 

 was much lower in the case of the negatively charged leaf. 



The discharge effect with the magnetic field became 

 apparent with pressures as high as '05 mm. of mercury. 

 It was found that when the leaf was charged below the 

 critical potential [so that discharge did not take place when 

 the field was put on] and the pressure was *0l mm. of 

 mercury, the magnetic field had not such a marked effect as 



