;>lt> Bumstead and M&Gougan — Emission of Electrons by 



the leaf can be insulated, grounded, or charged to any desired 

 potential ; the volt-sensitiveness was thus taken after each 

 reading. In the following measurements, the sensitiveness 

 was adjusted to give a deflection of ahout 25 divisions on 

 the scale of the microscope for - 2 volts. The tuhe T is con- 

 nected to pump, gauge, and charcoal bulb. 



When the o-rays fall on the electrode, E, they carry over to 

 it their positive charges; the S-rays which they excite are 

 negatively charged and these, leaving the electrode, add to 

 its positive charge. In order to insure the removal of all 

 the emitted electrons from the electrode and to prevent 

 the S-rays emitted by other parts of the apparatus from 

 reaching it, a positive potential of 40 volts is applied to 

 the case. If a sufficient magnetic field is applied with its 

 lines of force parallel to the electrode, the electrons emitted 

 will be turned back to the electrode, and in this way the 

 charge due to the a-rays alone may he determined. For this 

 purpose an electromagnet was constructed of Swedish iron, 2 

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

 rectangle 30 cm by 25 cro and a gap left in one side lS^ ™ 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 5 am- 

 peres 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 

 represent the number of aluminium foils between the polonium 

 and the electrode, and the abscissae are the currents measured 

 by the electroscope. Curve la 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 lb is obtained by subtracting the abscissae of II 

 from la and represents the 8-ray effect, or "ionization" of the 

 metal. 



It will be observed from an inspection of Curve II that the 

 number of a-particles which reach the electrode apparently 

 decreases as more foils are interposed. Up to five thick foils 

 this decrease is approximately linear, and amounts to about 20 

 per cent of the total. This is too great a falling off to be 

 attributed to the scattering of the a-rays according to the 



