MAGNETIC METHODS 



99 



The commutator consists of two brass half rings which surround the 

 lower end of the axis of the coil. The half rings are connected to the 

 ends of the wire of the coil but are insulated from the coil axis and from 

 each other. The commutator brushes, one on each side of the axis of the 

 coil, are attached to the large ring but are well insulated from it. They 

 are so placed that commutation occurs when the plane of the coil is parallel 

 to the inclination axis. The alternating current produced in the rotating 

 coil by the induced E.M.F. is conveyed to the half rings of the commutator, 

 taken off by the brushes as a direct current, and carried by the attached 

 leads to the galvanometer. 



Fig. 30. — Field magnetometer set up for timing os- 

 cillations for horizontal intensity and observing decli- 

 nation: A, torsion tube; B, thermometer; C, stirrup 

 (magnet holder); D, magnet arrester; E, control for 

 auxiliary lens; F, telescope; G, horizontal circle; H, 

 slow motion screw; I, lower motion clamp; J, slow 

 motion screw; K, tripod head; L, magnet; M, mirror; 

 N, iris diaphragm. 



The operation of the instrument for dip determinations consists in 

 placing the axis of the coil in the plane of the magnetic meridian and al- 

 tering the inclination of the coil axis (by turning the ring) until a position 

 is found where the galvanometer indicator stays at zero when the coil is 



