470 Prof. Bumstead and Mr. McGougan on Emission of 



back to the electrode, and in this way the charge due to the 

 a-rays alone may be determined. For this purpose an 

 electromagnet was constructed of Swedish iron, 2 inches 

 square in section. It was forged into the shape of a rect- 

 angle 30 cm. by 25 cm., and a gap left in one side 15*2 cm. 

 long which was just sufficient to embrace the exhausted 

 chamber. It was wound with about 1000 turns of No. 14 

 cotton insulated, paraffined wire. The field at various points 

 between the poles was measured with a Grassot flux-meter ; 

 a current of 3 amperes produced a field, midway between the 

 poles, of 95 gausses. It was found that, with the case 

 earthed, this field reduced the current received by the 

 electrode to a minimum ; no further diminution occurred 

 when the current through the magnet-coil was increased to 

 9 amperes. On account of the large air-gap the field was 

 very nearly proportional to the current. 



The results obtained when the electrode was a sheet of 

 aluminium are shown in fig. 2, in which the ordinates repre- 

 sent the number of aluminium foils between the polonium 



(lb S i I /, i a 



/ i 





and the electrode, and the abscissae are the currents measured 

 by the electroscope. Curve I. a gives the results with no 

 magnetic field, and thus represents the total effect due to both 

 a- and S-rays. Curve II. shows the currents observed when 

 the magnetic field was on, due to the charge carried by the 

 a-rays alone. Curve I. b is obtained by subtracting the 



