On a Wehnelt Cathode-Ray Tube Magnetometer. 381 



the analogous formula of Ramage and may perhaps be 

 justified on similar grounds. The interesting point, however, 

 is that this formula does what no other equation suggested 

 has succeeded in doing. It leads the series right down so as 

 to include the boiling-point of hydrogen as the zero paraffin. 

 The wish to do this constituted the original incentive to the 

 work contained in this paper. It was suggested by reading 

 the two papers by my colleague, Prof. Sydney Young, to 

 which reference has been made and from which all the 

 experimental material used above has been taken. To his 

 kindly interest this paper is entirely due. 



Dunsink Observatory, 

 July 31, 1916. 



XLII. A Wehnelt Cathode-Ray Tube Magnetometer. By 

 Chas. T. Knipp, M.A., Ph.D., Associate Professor of 

 Experimental Electricity, and L. A. Wblo, M.S., Research 

 Assistant in Astronomy, University of Illinois, U.S.A.* 



[Plate VIII.] 



I. Introduction. 



IN a recent paper f the authors described an apparatus for 

 determining the horizontal intensity (H) of the earth- 

 magnetic field, depending on the measurement of the mag- 

 netic and electrostatic deflexions of a beam of cathode rays 

 and on the assumption of the ratio of the charge to the mass 

 of the electron. It has since been suggested to the authors 

 that it might be well to deduce H by comparing the de- 

 flexion produced by the earth's magnetic field with the 

 deflexion produced by a known calculated field, thus 

 avoiding the assumption of the ratio e/m. It was for the 

 purpose of carrying out this suggestion, and also in the hope 

 of obtaining a more compact and convenient instrument, that 

 the work now to be described was undertaken. 



II. The Apparatus and the Manipulation. 



The discharge-tube is a glass jar about 40 cm. in height 

 and of 12 cm. diameter, clamped at the bottom to a brass 

 plate, which is in turn fastened to the widened part of a 

 brass pipe about 5 cm. in diameter. The last fastening is 



* Communicated by the Authors. 



t Knipp and Welo, Terrestrial Magnetism and Atmospheric Elec- 

 tricity, vol. xx. 1915, pp. 53-68. 



