382 Prof, C. T. Knipp and Mr. L. A. Welo on a 



adjustable so that the axis of the discharge-tube can be made 

 to coincide exactly with that of the pipe, which turns in 

 holes cut in a wooden frame. The pipe thus serves as a 

 bearing, permitting the discharge-tube to be turned about 

 a vertical axis. The rotation about the vertical axis is a 

 necessary feature of the instrument, for the earth's magnetic 

 field cannot very well be eliminated for a zero reading 

 of the deflexions, but they must be observed for various 

 orientations of the tube. 



The tube is exhausted through a glass stem inside of the 

 brass pipe. One end of the stem is waxed into a hole at 

 the bottom of the jar, and the other is fitted by a long and 

 well-ground joint to another glass stem which connects to a 

 Gaede capsule-pump, to a bulb containing the drier P 2 Oo, 

 and to the charcoal-liquid-air bulb for further improving 

 the vacuum. 



The cathode shown at C, fig. 1, is of a form previously 

 described by the authors * but has, in this case, been some- 

 what simplified and made more compact. The object of 

 using the Wehnelt cathode rather than the cold aluminium 

 cathode is to secure electrons having a low velocity with 

 consequent measurable deflexions for the relatively weak 

 earth-field, and at the same time get a definite beam of 

 sufficient range without the use of diaphragms, as was 

 found possible when the spot of lime (Bank of England 

 wax) does not exceed 0*02 cm. in diameter. The direction 

 that the beam takes is uncertain, and two adjustments of the 

 cathode are necessary, one in the plane of the drawing and 

 one perpendicular to it. If the beam should take some 

 such direction as that indicated in the drawing, fig. 1, it can 

 be made to take a vertical direction by turning at the joint 

 J 2 , and a similar adjustment in a plane perpendicular to the 

 plane of the drawing can be made by turning at J 3 . 



The anode is an aluminium ring of nearly the internal 

 diameter of the jar, lying at its bottom, and connected by a 

 wire to mercury at the bottom of the glass stem below the 

 joint J x . The mercury is in contact with a sealed-in platinum 

 wire which is connected to the positive terminal of a high 

 potential battery. Since the cathode and the wire leading 

 to the anode from the inside of the exhaust-tube are very 

 close together, a mica screen is inserted at M to prevent a 

 direct discharge which might take place if the vacuum 

 were at a favourable stage, with possible injury to the 

 battery. 



The deflexions of the beam of particles are recorded on a 

 * Loc. cit. 





