2l8 MEASUREMENT OF CURRENTS. 



840. ARTIFICIAL FIELD. Several arrangements may be used 

 to diminish the action of the external field. If the bar is placed 

 symmetrically in repect of the axis of rotation of the needle, and 

 all the points are at a great distance from the needle, its magnetic 

 field is sensibly uniform and horizontal in the space it occupies. 



If H' and H are the horizontal components of the field of the 

 bar and of the terrestrial field, and the angle of their respective 

 directions, the intensity R of the resultant field and the angle a 

 which it makes with the meridian are given by the equation 



(H + H' cos (9) tana = H' sin<9. 



The minimum of R, which corresponds to O = TT is (H-H'). 

 If the ratio of the fields H and H' is near unity, the angle a is near 



- . It tends towards - when approaches TT that is to say, when 



the position of equilibrium of the needle tends to make a right angle 

 with the meridian, as for a system of quasi-astatic needles (299). 



The apparatus is then very sensitive to variations in intensity or 

 in direction of the component fields. Thus we have the changes </H 

 of the terrestrial field, 



H'cos0_ <m 



TT" -w--* u > = ^^r' 



and for changes in direction 



do. H'(Hcos0 + H') H' 



dB R 2 H - H' 



In the condition of maximum sensitiveness, the difference H - H' 

 being very small, the relative variation of the resultant field is very 

 great, and the position of equilibrium changes considerably with 

 variations in the declination. 



The compensating magnet is usually supported by a vertical rod 

 placed on the galvanometer in the prolongation of the axis of 

 rotation of the needle. The magnet can be fixed at any given 

 height by means of a screw, and by a tangent screw the rod 

 itself may be turned about its axis. The magnet has the form 

 of the arc of a circle, so as to have a more uniform field in the 



