HEYL & BRIGGS— THE EARTH INDUCTOR COMPASS. 27 



the earth's field the conditions differ in one important respect from 

 those obtaining in the dynamo; in the earth's field we must reckon 

 with free poles in the armature, and a consequent self-demagnetizing 

 effect. This effect is a minimum if the iron is in the form of a rod 

 long as compared to its diameter. This leads to a cross-shaped arma- 

 ture 5, as shown in the photograph. The arms are of 3^ per cent, 

 silicon steel, which has, in a field equal to the earth's horizontal com- 

 ponent, a permeability of about four times that of ordinary soft iron. 

 The residual magnetism is also considerably less. The four arms of 

 the cross are carried by a central hub of the same material. The 

 arms are i cm. in diameter, and measure 20 cm. from end to end of 

 a pair of opposite arms. For a rod of these proportions the demag- 

 netizing coefficient is small enough to allow a permeability of about 

 ten times that of a cube of the same material. 



Upon each of the four arms there are 500 turns of No. 20 B. & S. 

 copper wire, silk enamel insulation. The winding is of the closed 

 coil type, with a four-segment commutator. The total resistance of 

 the wire on all four arms is 3.2 ohms, and the resistance through a 

 pair of opposite commutator segments 0.8 ohm. At 20 revolutions 

 per second, the electromotive force is 8 millivolts. 



The armature and commutator are carried by the axle 6, which 

 runs in a thrust ball bearing mounted in the gimbal rings 7. There 

 are four collecting brushes of carbon, spaced 90° apart, on a mount- 

 ing which swings with the axle and commutator. By turning the 

 whole gimbal system by means of the slots and screws 8 the brush 

 system can be set at any desired angle with respect to the vessel in 

 which it is installed. The resistance of the armature through a pair 

 of brushes is from i to 1.5 ohms. 



Below the armature 5 is a brass weight p, weighing about a kilo- 

 gram. The length of the pendulum thus formed is short enough to 

 give a time of (half) swing of about one third of a second. The 

 bearings for the gimbal rings 7 are provided with leather friction 

 washers, by tightening which any desired degree of damping may be 

 applied. It is usual to damp the pendulum so that it will execute 

 from six to eight half-swings before coming to rest after a displace- 

 ment of about 20°, occupying from two to three seconds in the 



