68 Mr. Peck on the Effect of Transverse Magnetic Field 



than they normally possess. Hence the resistance in the 

 dark space would be diminished. 



According to this view, the reduction o£ the resistance 

 caused by a transverse magnetic field at the cathode depends 

 on the existence of the normal large fall of potential near the 

 cathode ; therefore anything which does away with the 

 cathode fall of potential should also eliminate, in whole or in 

 part, the effect noticed by Dr. Willows, and cause a trans- 

 verse magnetic field in this region either to reduce the resist- 

 ance less than it generally does or to have no effect, or finally 

 increase it as it does at other parts of the tube. 



Now Wehnelt"* has found that when the cathode is coated 

 with certain metallic oxides (calcium oxide for example) and 

 then raised to incandescence, the cathode fall of potential is 

 very largely reduced. It therefore seemed desirable to 

 investigate the effect of a transverse magnetic field on the 

 resistance in a vacuum-tube when the cathode was heated and 

 coated with calcium oxide. 



The results of the experiments are given below. 



The Apparatus. — The vacuum-tube contained air, and was 

 supported vertically with the cathode at the bottom. The 

 cathode consisted of a small inverted U-loop of platinum wire 

 joined on to stouter copper leads. The cathode was heated 

 by joining the leads to a battery ; an adjustable resistance 

 being placed in the circuit. The platinum was coated with 

 calcium oxide by alternately dipping it into a strong solution 

 of calcium nitrate and heating in the blowpipe-flame. When 

 prepared, the cathode leads were cemented into the lower 

 end of the glass tube with sealing-wax. The lower part of 

 the tube was surrounded with cold water to prevent the wax 

 from melting when the heating-current was passed through 

 the copper leads to the cathode. 



An electromagnet was arranged with its pole-pieces level 

 with the cathode, and the direction of the magnetic lines of 

 force was parallel with the plane of the U-loop. 



The cathode was earthed, and current passed through the 

 tube from an induction-coil. 



The variations in the potential-difference between the 

 electrodes of the tube were measured with a Kelvin electro- 

 static voltmeter. A tube containing xylene was joined up 

 to the induction-coil in parallel with the vacuum-tube, 

 and one terminal of the voltmeter was connected with an 

 end of this tube while the other terminal was connected with 

 the xylene at a suitable point between the ends of the tube so 

 as to give a readable deflexion on the voltmeter. In every 

 * Ann. d. JPhysik, xiv. 3, pp. 425-468. 



