10 Land Magnetic Observations, 1911-13 



same time, serves effectually to keep any currents of air out of the magnet house. The 

 magnetometer attaclmient, including its telescope and suspension, is packed assembled 

 and may be mounted on the base in one operation by a device similar to that described 

 for the theodolite-magnetometer. 



The inductor and theodohte attachment (see Plate 3, Fig. 1, and Plate 4, Fig. 1) 

 consists of a U-shaped standard frame supporting an inner ring in which are provided agate 

 bearings to carry the inductor coil; the telescope and vertical circle are mounted on 

 the axle ends of this ring outside of the standards. The center of the telescope is 78 mm. 

 from the center of the instrument. The telescope has a magnifying power of about 17. 

 Suitable shade glass and a prismatic eyepiece are provided for astronomical observations. 

 The reticle is a small cross of two fine fibers at right angles. The vertical circle is 102 mm. 

 in diameter and has the same style of graduation as the horizontal circle, being read directly 

 to one minute of arc by two verniers and by estimation to one-quarter minute. The circle 

 is attached to the axle of the bearing ring in such a way that the relation of the line of 

 sight of the telescope to the zero graduation of the vertical circle is constant, and the vertical 

 circle rotates with the telescope. The two verniers are fixed in position by suitable frame- 

 work attached to the standards of the instrument. The graduated circle is protected by 

 a sheath of brass with uncovered openings at the verniers. To protect the circle and 

 verniers against tarnishing, they are coated with lacquer. 



The ring carrying the bearings for the coil is oval-shaped, the inner smaller diameter 

 being 80 mm. and the inner greater diameter about 103 mm. The section of this ring is 

 about 8 mm. by 10 mm., with suitable lugs cast on at the points where the axle supports 

 and coil bearings are placed. The bearings for the rotation axis of the coil are of agate, 

 the bearing surfaces being cone-shaped. The bearing at the commutator end is the smaller; 

 the one at the opposite side is larger, to permit the drilling along the center axis of a hole 

 of sufficient diameter (4 mm.) to permit of making attachments for the pin connection for 

 the flexible shaft used for rotating the coil. The cap carrying the agate bearing at the 

 conmiutator end is provided with adjustments for careful centering, while that carrying 

 the opposite bearing has a longitudinal adjustment along the axis to permit the taking up 

 of any slight wear or lost motion in the beai'ings. 



The coil is mounted on a hard-rubber spool 24 mm. thick; the inner diameter of the 

 spool is 26 mm. and the outer 74 mm. The winding consists of 0.23 kilogram of double- 

 wound silk magnet wke. Brown and Sharpe gage No. 28, in 49 layers and 2,062 turns; 

 at every thu-d layer of wire there is a double layer of paper; the width of the winding is 

 18 mm. and the outer diameter 71 mm.; the resistance of the coil is 68 ohms. The coil is 

 covered with a heavy coating of paraffin to protect it against possible abrasion and short- 

 circuiting of the turns and against moisture. The ends of the phosphor-bronze rotation 

 axis for the coil spool are attached to the hard rubber by means of two U-shaped brass 

 straps. The commutator is mounted on a core of fiber. The brushes are of hard copper 

 attached to the brass supporting-ring by screws and insulated from it by hard-rubber 

 sleeves and washers, and connected to insulated wires running in grooves in the ring and 

 terminating at the binding posts (Plate 4, Fig. 1) at the extremity of the axle on the side of 

 the vertical circle of the instrument. Suitable means for clamping the vertical circle and for 

 slow motion are provided. For the purpose of facilitating more rapid approximations, a 

 micrometer head is attached to the slow-motion screw and a suitable index provided; 

 motion through three divisions on this head corresponds approxunately to one minute of 

 arc on the vertical circle. 



The magnetic meridian may be determined by direct and reverse sightings with the 

 telescope on a mark of known magnetic azimuth. As this method, however, is not always 

 feasible, a compass (see Plate 4, Fig. 1) is added for determining the magnetic meridian. 

 Mounting for the compass is provided on the ring supporting the inductor coil, the ring 



