Discharge of Electricity through Vacuum- Tube. 



67 



with the under surface of its conical head. On account of 

 the angular momentum the effect of this is, not to turn the 

 point of the bullet upwards,, but to deflect it very slightly to 

 the right (in the case of left-handed rifling, to the left), 

 because the axis of the very small angular momentum im- 

 pressed upon the bullet in one revolution points horizontally 

 to the right. The bullet now begins to experience an excess 

 of pressure upon its left front, upon the whole in a horizontal 

 plane. Since the angular momentum thus imparted in one 

 revolution has its axis directed vertically downwards (N.B. 

 every diameter of the bullet through the C.GL which is at 

 right angles to the axis is a principal axis of inertia) the 

 bullet is now compelled to turn its point downwards, and to 

 bring its axis into parallelism with the tangent to the path 

 of its G.G. ; and the disturbing couples steadily decrease as 

 the axis approaches this position. Should the axis dip below 

 the tangent, similar actions will at once begin to bring it up 

 again. The pressure upon the left front of the bullet must 

 deflect the trajectory slightly to the right. 

 Woodroffe, Bournemouth. 



A 



VJII. On the Effect of a Transverse Magnetic Field on the 

 Discharge of Electricity through a Vacuum-Tube. By 

 Jobn Peck, B.Sc. * 



S is well known, the general effect of a transverse mag- 

 netic field on the discharge through a vacuum-tube is 



o jo 



to produce an increase in the potential-difference of the 

 electrodes ; this being probably due to the deflexion of all 

 the ions to one side of the tube so as to diminish its effective 

 section and thus increase the resistance in the gas. 



It has, however, been shown by Dr. Willows f that a trans- 

 verse magnetic field acting at the cathode reduces the 

 resistance in the tube if the pressure of the gas is below a 

 certain value, although it increases it at higher pressures. 



Quite recently J the same investigator has suggested that 

 this reduction of the total resistance in the tube is caused by 

 the magnetic field decreasing the fall of potential taking place 

 near the cathode owing to the deflexion of the cathode rays, 

 which, instead of passing directly through the dark space 

 with great velocity, are now bent back so that they have a 

 much longer path, and therefore a much better chance of 

 producing fresh ions by collision, in the cathode dark space 



* Communicated bv the Author. 



t Phil. Mag. Feb. 1901. 



t Phys. Proc. Jan. 27, 1905. 



F 2 



