Mr Almy, On a Diminution of the Potential Difference, etc. ]83 



On a Diminution of the Potential Difference between the Elec- 

 trodes of a Vacuum Tube, produced by a Magnetic Field at the 

 Cathode. By John E. Almy, University of Nebraska. 



[Read 6 May 1901.] 



Characteristic effects of a magnetic field upon the discharge 

 in vacuum tubes, especially with large field strength at the 

 cathode have been noted and studied by Birkeland, Melani 1 , and 

 others. Birkeland found that with the lines of force parallel to 

 the direction of the discharge, at gas pressures below 0'012 mm. 

 of mercury, the P.-D. between the electrodes was made to diminish 

 by the magnetic field, at first rather slowly as the magnetic in- 

 tensity increases ; when however a certain " critical intensity " is 

 reached a large, abrupt, increase of this diminution is obtained 

 with a very small increase of magnetic intensity. Further study 

 of this effect was my purpose. 



I found that not only fields parallel to the discharge, but also 

 a field normal to the direction of discharge, gave this large 

 and rather anomalous diminution of potential difference, and in 

 fact the so-called " critical " intensity was much smaller in the 

 latter case. 



Using an induction coil giving a 6-inch spark to furnish the 

 discharge, a spark gap, with two brass spheres in shunt to the 

 tube to measure the potential difference between electrodes, 

 a solenoidal electro-magnet with soft iron core 4 cm. in diameter, 

 about 40 cm. long, to give the magnetic fields, the effects pro- 

 duced in a tube 3o cm. in diameter, 20 cm. in length, having 

 plane aluminium electrodes (discs), were observed. The following 

 series, with gas pressure of 009 mm. of mercury, are typical : 



1 Cf. Birkeland, Comptes Rendus, Vol. 126, p. 586 ; Melani, Nuovo Cimento 

 (Series 4), Vol. 5, p. 329. 



