14 Land Magnetic Observations, 1911-13 



The inside diameter of the winding of the coil is 26 mm., the outside diameter, 73 mm., 

 and the width of winding, 17.5 mm. There are 65 layers (3,162 turns) of double-wound 

 silk magnet wire, No. 30 Brown and Sharpe gage, with a double thickness of paper at every 

 fifth layer. For protection against moisture conditions encountered on board ship and 

 against possible abrasion and short-circuiting of the turns, the outer surface of the coil and 

 its connections are heavily coated with paraffin. The resistance of the coil is about 175 

 ohms. 



For the purpose of determining the magnetic meridian, a sighting telescope and a 

 compass are provided, suitable mounting wyes being placed so that the line of sight or 

 the magnetic axis of the needle will be in the vertical plane through the rotation axis of 

 the coil. The magnetic meridian may thus be determined by sighting upon marks of 

 known magnetic bearing or by actual observation of the compass. Parallax in the compass 

 readings is avoided with the aid of mirrors mounted inmiediately below the ends of the needle. 



The gearing for rotating the coil of the earth inductor is self-contained. A hole has 

 been drilled through the center of the spindle and a shaft mounted in it with a miter gear 

 at the upper end in suitable bearing ; this engages a second miter gear mounted on a shaft 

 set at about 45 from the vertical in fixed bearings on the standard frame. A third gear 

 at the upper end of the inclined shaft engages a similar miter gear attached to an axle 

 rotating in the center of the horizontal bearing-end of the supporting ring. Inside of the 

 supporting ring there is attached to this axle a gear of 102 teeth mounted in a hollow 

 spherical frame which permits the coil to turn freely inside it, and engages a gear of 25 

 teeth attached to the rotation axis of the coil. 



For the purpose of rotating the coil at land stations, a special tripod clamp has been 

 provided with a bearing for turning the crank which is connected with the center shaft 

 in the spindle bearing by means of brass shafts and two universal joints. The weight of 

 the present earth inductor is about 5 kilograms, while that of the Wild-Eschenhagen 

 pattern is about two and a half times as much. The comparisons of it with the standard 

 earth inductor of the latter pattern have been extremely satisfactory, the resulting difference 

 being on the order of about 0.1 minute. 



The galvanometer used is of the moving-coil type, manufactured by the Leeds & 

 Northrup Company, of Philadelphia, with some slight modifications specified by the 

 Department of Terrestrial Magnetism. The moving coil is suspended between straight 

 upper and lower suspensions of 0.002-inch phosphor-bronze ribbon. These suspensions 

 have proved to be amply strong for carrying the instrument about and for the more or less 

 rough motions encountered during heavy seas on board ship. The coil may be clamped 

 in position when not in use by means of a sliding clamp at the back of the tube and the 

 tension of the suspension fibers adjusted by a sliding rod and set-screw at the lower end 

 of the tube. The tube containing the coil and suspensions may be quickly removed by 

 loosening two clamping nuts and readily replaced by a reserve tube, if necessary. A glass 

 window allows easy inspection of the system. The reflecting mirror has a plane surface. 

 The galvanometer resistance is 317 ohms and the critical damping resistance about 125 

 ohms. The deflections are as nearly as possible proportional to current and the sensibility 

 practically the same for negative deflections as for positive ones. The sensibility specified 

 was 1 nam. at scale distance 1 meter =10"* ampere. The period is about 2.4 seconds. 

 To prevent corrosion and warping, owing to the hygrometric conditions at sea, the magnet 

 and core are protected by paint and the base is made of hard rubber. The stray magnetic 

 field was specified to be as small as possible. This galvanometer has been used for the 

 land determinations by the observing staff of the Carnegie, care being taken always to see 

 that it is sufficiently removed from the observing station to occasion no distortion of the 

 Earth's magnetic field. 



Inductor No. 4, made in the shop of the Department of Terrestrial Magnetism, and 

 illustrated in Plate 4, Fig. 3, is in general detail entirely similar to the marine type just 



