Descriptions of Instruments 13 



EARTH INDUCTORS. 



The types of earth inductor used were: (a) the design originated by Wild' and as 

 modified by Eschenhagen, described and ilhistrated in Vohime I and represented in the 

 Department's equipment by No. 48 constructed by Schulze, and No. 2 constructed by 

 Toepfer & Sohn; (6) earth inductor of the type made by the Department of Terrestrial 

 Magnetism for the determination of dip at sea and as represented by earth inductors Nos. 

 3 and 4; earth-incUictor attachment of universal magnetometer of type 4(c), described 

 and illustrated in this volume.^ 



Type (6) is the style of instrument originally made by the Department of Terrestrial 

 Magnetism for observations at sea. The elements of the apparatus as used at sea are 

 essentially: (1) an improved form of gimbal stand that will maintain an average mean 

 position of equilibrium but which permits complete reversal of the gimbal rings and bearings 

 in order to eliminate errors of level (the description of this gimbal stand will be given in 

 a later volume containing the detailed results of the observations at sea; in the meanwhile 

 the interested reader may be referred to the journal Terrestrial Magnetism, vol. 18, pages 

 40 and 41); (2) a portable form of earth inductor with such means for rotating the coil 

 as will not in any way when in use disturb the gimbal rings; and (3) a galvanometer of 

 sufficient sensibility suitable for use at sea. 



The earth inductor constructed for this apparatus is shown in Plate 4, Fig. 2. The 

 double-center base is of the usual pattern used for theodolites, with three leveling foot- 

 screws and slow tangent-motions for each center. The graduated horizontal circle is 

 120 mm. in diameter, the least graduation being 30 minutes of arc ; it may be read, by means 

 of two verniers, directly to one minute of arc, the very sharp, clean graduations permitting 

 estimations 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 simply because it was in stock, 

 and thus made possible the early completion of the instrument. The standard carrying 

 the ring in which the coil is mounted and on which the rotating gear and the vertical-circle 

 verniers are also mounted, is attached permanently to the base. The ring carrying the 

 bearings for the rotation axis of the coil is 78 mm. 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 with suitable wire connections 

 to two binding posts through the horizontal axle on the vertical-circle side of the instrument. 

 To make possible the more accurate adjustment of the brushes, it was originally contem- 

 plated to make them adjustable around the axis of the commutator, but this was not done 

 because of the mechanical complications that would have been introduced. Care was used, 

 however, to set the brushes very closely to eliminate the necessity, for practical purposes, 

 of this adjustment. The vertical circle is 102 mm. in diameter, with a least graduation 

 of 30 minutes of arc; it may be read directly, by two fixed verniers, to one minute and, 

 by estimation, to one-quarter of a minute. Suitable means for clamping vertical circle 

 and for slow motion are provided. 



The bearings of the rotation axis of the coil are of brass, being V-shaped in longitudinal 

 section and running in agate cups burnished in the brass centering supports in the supporting 

 ring. The coil is held in place by two U-shaped pieces of brass which carry the bearing 

 ends of the rotation axis. At one end of the rotation axis is mounted the commutator 

 and, at the other end, a miter gear for use in the rotation of the coil. The spool of the coil 

 is made of hard rubber^ of 24 mm. outside thickness and of 74 mm. outside diameter. 



'Wild, H. Inductions-Inclinatorium neuer Construction und Bestimmung der Absoluten Inclination mit demselben 

 in Pawlowsk. St. Petersburg, Mem. Ac. Sc, Ser. 7, v. 38, No. 3, 1891. 



'For a special study on "The Theory of the Earth Inductor as an Inclinometer," made by Dr. N. E. Dorsey in 1912, 

 see his article in Terr. Mag., vol. 18, pp. 1-38. 



'This material was used instead of brass because of the greater mechanical ease of maintaining insulation and eliminating 

 induction effects. 



