302 Danysz and Duane — Electric Charges of a- and $-Rays. 



A number of measurements have been made to ascertain the 

 effect produced by superposing electric fields on the magnetic 

 field. In general, if the magnetic field is small, the electric 

 field produces a large change in the value of the currents. 

 Charging the cylinder B to +88 volts, for instance, increases 

 the current to four or five times its value, while, if the poten- 

 tial is —88 volts, the current is diminished about the same 

 amount, becoming negative. These changes are due, undoubt- 

 edly, to the drift produced by the electric field in the electrons 

 forming the secondary rays, in spite of the suppressing effect 

 of the magnetic field. 



On the other hand, the saturated current for large magnetic 

 fields is not changed more than a few per cent (1 — 8) by 

 electric fields up to 1800 volts. In general, the effect due to a 

 positive charge given to the cylinder B is somewhat greater 

 than that due to a negative charge. The effect of small electric 

 fields is small ; for instance, in one experiment with H = 8000 

 gauss, an electric field of + 2 volts increased the current 2*5 

 per cent, and a field of —2 volts decreased it *8 per cent, 

 When the cylinder B is put to earth the electric field is very 

 small, for both B and E are covered with sheets of the same 

 metal, aluminium, and we are safe in assuming that the maxi- 

 mum current is not altered appreciably by any small drift in 

 the electrons due to the Volta electromotive force. 



A number of experiments have been made to determine the 

 magnitude of the charge carried by the a-rays from a given 

 quantity of emanation. In a preliminary experiment, in which 

 the glass sphere containing the emanation was small, the posi- 

 tive charge did not decrease with the time as fast as it should 

 according to the law of decay of the emanation. This was prob- 

 ably due to a gradual shifting of the bubble of emanation in 

 the sphere, portions of the latter's walls being too thick to 

 allow all the a-particles to pass through. 



This progressive change did not take place in the two series 

 of experiments, the data of which is contained in the subjoined 

 table. In these experiments the spheres had much larger 

 volumes than the bubbles, and the currents followed very 

 closely the law of decay of the emanation with the time. 



The quantity of emanation contained in each of the glass 

 spheres was measured by comparing the ionization due to its 

 7-rays with that due to the 7-rays from a secondary radium 

 standard belonging to the laboratory of Madame Curie. This 

 standard had been compared previously with the international 

 primary standard. A correction was made for the difference 

 in thickness of the glass of the little sphere and that of the 

 tube in the radium standard, the magnitude of the correction 

 being estimated by covering a little bulb with a sheet of alu- 



