200 Ocean Magnetic Observations, 1905-16 



therefore, necessary to observe deflections for particular settings of the rotation axis of 

 the coil on either side of the line of magnetic inclination, taking care that a constant speed 

 of rotation is maintained. With the aid of chronometer beats,^ experiment has shown that 

 there is comparatively little trouble in holding the speed sufficiently constant for practical 

 purposes, account being taken of the desired accuracy of deflection determination for the 

 comparatively small displacements from the line of actual magnetic inclination. By 

 observing the mean deflections of the galvanometer for right-hand and left-hand rotation 

 of the coil, respectively, when the rotation axis is in the magnetic meridian, it is possible 

 to determine at once the position of balance, or the vertical-circle reading of the true line 

 of incUnation, by Unear interpolation from the vertical-circle settings used. There being 

 no means provided to determine directly the reading of the vertical circle when the rotation 

 axis is vertical, corresponding observations and mterpolations must be made with the verti- 

 cal circle turned in azimuth 180° from its first position. The graduation is continuous 

 through 360°, so that the difference between the two positions for balance so determined 

 gives twice the complement of the angle of magnetic inclination, from which the value 

 may be at once deduced. 



Credit is due Mr. J. A. Widmer, chief instrument-maker of the Department, and his 

 assistants, for the excellent execution of the mechanical detail. 



Installation on the Carnegie.^ 



The apparatus was installed on board the Carnegie at Papeete, Tahiti, in October 1912. 

 The inductor was mounted on the new gimbal-stand (PI. 14, Fig. 5) in the forward observa- 

 tory, B (see PI. 9, Fig. 2). In order to eliminate any possible magnetic effect arising 

 from the large field magnet of the galvanometer, it was necessary to place the galvanometer 

 at a considerable distance from the mounts for the magnetic instruments. During October 

 to November 1912 it was fastened by a bolt passing through its hard-rubber base to a 

 substantial shelf against a wood wall about 30 feet aft of the deflector. In December 1912 

 a small structure to house the galvanometer was built on the quarter deck just aft of the 

 engine-room skyhght. The galvanometer was then mounted on a shelf, about 49 feet 

 distant from the deflector. Communication between the observers at the earth inductor 

 and the galvanometer is estabhshed by means of a simple signaUng device. 



The chief difficulty at sea has been the maintenance of the adjustment of the galvano- 

 meter (see p. 201). It is feasible to adjust the system readily at shore stations so that a 

 tip of 20° in any direction, with a scale distance of 58 cm., causes a change of not more 

 than 1 nma. in the galvanometer zero. Fortunately, after some practice, it has been 

 found possible to make the adjustment for balance on board, even when the sea is moderately 

 rough, within 3 to 15 minutes, depending upon how badly the coil is out of balance and 

 upon the character of the ship's motion. In moderately rough seas, the wandering of the 

 zero is reduced to a range of 5 to 10 mm. when the galvanometer is used with a 100-ohm 

 shunt. The failure to maintain balance at sea is also partly due to the temperature and 

 humidity changes. Owing to the lack of symmetry in the balancing arrangements, it is 

 difficult to throw the center of gravity of the coil into the line of suspension. By manipu- 

 lation it has been found easier to place the center of gravity in a plane at right angles to 

 the coil than in the plane of the coil; for this reason the galvanometer has been mounted 

 with the plane of its coil fore and aft, since the roll of the ship is more effective than the 

 pitch. 



'Experimeats were also made with a metronome, but that instrument was unsuited for timing purposes on board ship. 

 ^From a report by C. W. Hewlett, Terr. Mag., vol. IS, pp. 46-48, 1913, with modifications. 



