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102

(6) Lord Kelvins Deflector provides a means of adjusting compasses when bearings of any kind are not available. It is designed upon the principle that there is no error in the compass when the directive force of the needles is the same with the ship’s head on any four points, each two of which are opposite to the other two. By means of the deflector, the directive force is measured with the ship’s head on any four points mutually at right angles. The directive force on these points is read successively upon the scale of the instrument and duly noted. The usual correctors are then applied so as to render the directive force equal on all points.

^7) Lord Kelvins Vertical Force Instrument (Fig. 7), for comparing the vertical force on board ship with the vertical force on shore; to enable the vertical magnets in the binnacle to be adjusted to correct the heeling error.

HI

FIG. 7.

The balanced magnet consists of two slightly curved steel slips, having their ends rivetted together, with their convex sides outwards. The lower of these slips is graduated on its under side, and carries a sliding weight which is free to move from end to end. The other steel slip is attached to a knife-edged axle, which is supported on agate V-bearings.

The containing case has a glass front and a brass roof carrying a spirit level on its upper side. The bottom has a long glass window to allow the slider and its scale-reading to be seen when the instrument is held upside down, as is done for adjustment or verification.

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