Marine Earth-Inductor 197 



knife-edges supporting an inner ring, on which are provided suitable grooves for footscrews. 

 The two free rings must, of course, be carefully balanced, and the inner one properly loaded 

 by a weight to insure a condition of stable equilibrium. It has been found in practice — 

 most probably because of the necessarily soft non-magnetic composition used for the 

 rather blunt knife-edges and the consequent wear — that the mean position of rest frequently 

 is considerably out of level and, furthermore, can not be relied upon to take up always the 

 same mean position. 



Accordingly, in the new gimbal stand designed by the Department, the soft metal 

 of the knife-edge supports was replaced by strips of hard alloy of platinum and iridium, 

 while the bearing pieces, mounted suitably in the gimbal rings, are of sUghtly curved agate. 

 (See Plate 14, Fig. 5, showing only part of the supporting frame; this frame has a wide 

 flange at the bottom, as in Fig. 1, to faciUtate bolting it in place.) The sUght curvature 

 of the agates insures, with a proper balancing-weight, that the bearings will be always 

 (except for violent displacements due to motion of vessel in heavy seas) along the same lines 

 of the agates. The outer or bearmg ring is attached rigidly to the stand, and is L-shaped, 

 with a groove at the lower edge to carry bell-metal balls providing a ball-bearing for the 

 load; there is another groove at the top for a second ball-bearing, which takes care of any 

 side thrust. A second ring rotates in these bearings, is centered as carefully as possible, 

 and is provided with a graduation on its upper surface at intervals of 1 degree; two 

 verniers fixed to the outer bearing-ring permit settings to be read directly to one-tenth of 

 a degree. The rotating ring may be clamped for any reading of the circle. It carries 

 two knife-edges of the platinum-iridium alloy, which support the larger and outer free 

 ring of the gimbal proper. The agate bearings of the outer gimbal-ring are mounted in 

 brass lugs on a diameter perpendicular to that in which smiilar knife-edges are provided. 

 The latter are in the same plane as the other knife-edges, and in turn support sunilarly 

 the inner gimbal-ring on which the instrument may be mounted. The material improve- 

 ment, in addition to that of the supports and bearings, is in the reversible feature for the 

 two gimbal-rings. A lead weight is mounted by four supporting rods from the inner ring; 

 this weight may be adjusted vertically by means of a screw-arm and lock-nuts. Sliding 

 clamps have been provided on the inner ring, so that the three grooved footscrews of the 

 earth inductor or other instrument used may be clamped in the V's; the clamping heads 

 and nuts are on the lower side of the ring and can not be seen in the figure. When the 

 stand is not in use, the gimbal rings are lifted off the agates and are supported by wooden 

 blocks and clamps. 



Earth inductor. — The earth inductor constructed for this apparatus is shown in Figure 

 3 of Plate 14. The double-center base is of the usual pattern used for theodohtes, with 

 three leveling footscrews and slow tangent-motions for each center. The horizontal- 

 circle graduation is 12 cm. in diameter; the least graduation is 30 minutes of arc. Read- 

 ings may be made, by means of two verniers, directly to 1 minute and by estimation, 

 because of the sharp, clean-cut graduations, to one-quarter of a minute. The lower 

 center of the base is not necessary, but for this inductor the double-center base was used 

 because it was in stock, and thus made possible the early completion of the instrument. 

 The standards carrying the ring in which the coil is mounted and on which the rotating 

 gear and the vertical-ckcle verniers are also mounted is attached permanently to the base. 

 The ring carrying the bearings for the rotation axis of the coil is 78 nam. inside diameter; 

 in it, at right angles to the axle supports in the standards, are provided centering and bearing 

 agates, and to it are attached the vertical circle and the commutator brushes. The wire 

 connections from the brushes are through the horizontal axle to two binding posts on the 

 vertical-circle side of the instrument. To make possible the more accurate adjustment of 

 the brushes, it was originally contemplated to make them adjustable around the axis of the 

 commutator, but, because of the mechanical compUcations that would have been intro- 



