Marine Earth-Inductor 190 



bearing of the base of the earth inductor engages in a slot in the upper part of the universal 

 joint, just visible in Plate 14, Figure 5. At the intersection of the two diameters containing 

 the supporting knife-edges there is no motion relative to the instrument; there results, 

 thus, no effect upon the position of the gimbal rings when the coil is spun. Theoretically, 

 of course, the universal joint should be a point, and accordingly it has been made as small 

 as is possible for practical use; the tests show that, with the gimbal weight mounted as on 

 board ship, no effect is produced on the average position of the gimbal rings. The gears 

 used are all of the same size, except the hollow-frame one in which the coil rotates; the 

 multiplication of rotation is in the ratio of 102 to 25, or approximately 4 to 1. The 

 wires from the earth-inductor binding posts to the galvanometer leads are very fine and 

 are made with long spirals to ehminate any effect on the position of the gimbal ring. 



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 kgm., while that of the Wild-Eschenhagen pattern is 

 about 2.5 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(3. 



Galvanometer. — The galvanometer used is of the moving-coil type, manufactm-ed by 

 the Leeds and Northrup Company, of Philadelphia, with some slight modifications specified 

 by the Department. 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 on board ship during heavy seas. The coil may be clamped in position when 

 not in use by means of a shding clamp at the back of the tube, and the tension of the sus- 

 pension fibers may be adjusted by means of 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 may be 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 is practically the same for minus deflections as for positive ones. The 

 sensibility specified was 1 mm. 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. (See PI. 14, Fig. 4.) 



The coil may be balanced by means of balancing-nuts mounted at its lower end so as 

 to make the zero reading practically independent of changes in level of the instrument up 

 to 20°. Owing primarily to defective centering of balancing rods and nuts, it has proved 

 somewhat difficult to balance the coU of the galvanometer first suppUed with the apparatus, 

 and the system is liable to become unbalanced in a short time. A new galvanometer in 

 which these mechanical defects have been eUminated was accordingly supplied to the Car- 

 negie for use, beginning at Port Stanley, Falkland Islands, in February 1913. It appears 

 that for the work with the marine earth-inductor during October 1912 to October 1916, the 

 absolute accuracy of observed values of inchnation depends largely upon the performance 

 of the galvanometer. A new marme galvanometer is being designed and constructed by the 

 Department of Terrestrial Magnetism, and it is expected that several suggested unprove- 

 ments, which may increase the accuracy of galvanometer readings on board, can be effected. 

 Method of observation. — The null method of observation, that is to say, the detei-mina- 

 tion of the position of the i-otation axis of the inductor coil for no current indication in the 

 galvanometer, commonly in use on land, can not, of course, be used on board ship. It is. 



