Wehnelt Cathode-Ray Tube Magnetometer. 385 



When the bulb has cooled it is immersed in liquid air. The 

 heating circuit may soon be closed, the resistance adjusted 

 until the platinum glows with a red heat, and 1000 volts now 

 applied between the cathode and anode. It is well to leave 

 the glowing cathode thus for, say, five minutes. If the 

 heating current is then slowly increased a diffused spot will 

 show on the plateholder-lid which is covered with willemite. 

 On further heating, the diffused spot becomes either a well- 

 defined spot or a sharp line in a plane containing the 

 platinum strip. After the cathode has been adjusted so 

 that the spot is east of the centre of the plateholder by an 

 amount equal to the estimated deflexion (and in the plane of 

 the coil), and the orientation of the tube noted, the plateholder 

 is opened and an exposure made for 5 to 15 seconds, de- 

 pending on the intensity of the beam. The switch of the 

 coil circuit is then closed so that the beam is deflected still 

 more, and the exposure at the new spot made. Then the 

 plateholder is closed, the tube turned to 180°, the plate- 

 holder again opened and the process repeated. We thus 

 get four spots or lines on the plate, which should be arranged 

 along a straight line. Half of the distance between the two 

 inner spots is the deflexion due to the earth's field alone, 

 while half the distance between the outer two is the deflexion 

 with the earth's field increased by the standard one. 



III. The Declination. 

 It is unnecessary to determine the declination by another 

 instrument in order to set the coil in a plane perpendicular 

 to the meridian. The instrument itself is indeed a suffi- 

 ciently accurate declinometer for the purpose of setting 

 the coil. 



Fig. 2. 





A test photographic plate is first taken with the coil in 

 any position, which is noted on the graduated circle, and 

 four spots obtained. Fig. 2 represents the general case. 

 OA is the deflexion due to the earth's field alone, and 

 OC that due to both fields. But 00 is the resultant of OA 



Phil, Mag. S. 6. Vol. 32. No. 190. Oct. 1916. 2 D 



