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



and the component AC of the coil-field making an angle $ 

 with BA produced. On this test-plate, then, the line BA is 

 drawn and produced; draw the line AC and measure the 

 angle (/>. The coil must be turned through the angle </> in 

 order that it be perpendicular to the magnetic meridian. 



IV. The Formula. 



Fig. 3 shows the relative positions of the cathode C which 

 is taken as the origin, the photographic plate P at a dist- 

 ance Xi, the centre of the coil at a distance r, the undeflected 



Fig. 3. 



path of the beam along the diameter taken as the x axis and 

 the deflected paths, with and without a current in the coil. 



Calling z e the deflexion produced by the earth's magnetic 

 field alone, the equation * 



e C Xx 

 .= — 1 H(#i— x)dx . . . . 

 mvj 



(1) 



holds when the deflexions are so small that \-j~) may be 



neglected. When a current flows in the coil so that the two 

 fields have the same direction the resultant field is 



H + IH C . 



* J. J. Thomson, Phil. Mag. vol. xviii. pp. 844-845 (1909). 



