1851 Great Exhibition: Official Catalogue: Class X.: William Wilton
402. WILTON, WILLIAM, St. Day, Truro — Manufacturer.
Fox's magnetic dip and intensity instruments, various sizes, for showing the exact dip of the magnetic needle, and for measuring relative magnetic intensities. See the annexed cut.
This instrument consists of the following parts:-
1st. Of a horizontal plate, traversing within a graduated limb mounted upon a tripod of brass, and adjusted by screws to the true level, by a ground-glass spirit bubble.
2nd. Of a strong ring fixed at right angles to the former, within which are two graduated limbs, the outer one divided to 15°, and the inner one to 30°, and so fixed to the instrument, as to direct the eye in reading off from the points of the needle, thereby avoiding error from parallax.
3rd. Of a solid back with a graduated limb outside, and an armature carrying a telescope, with verniers made to revolve thereon.
4th. Of a concentric ring, revolving in the centre of this, and carrying a plate with bracket and appendages for the needles.
5th. Of a magnetic needle attached to a strong steel axle, terminating at the extremities in very fine, but perfect cylinders of hardened and polished steel; these fit into jewelled holes, which may be moved around with the concentric ring, and which suspend the needle in a vertical plane, making it very steady, yet allowing it perfect freedom to settle in the magnetic dip.
6th. Of a small brass stud projecting behind, and so fixed as to form a continuation from the axle of the needle; against this, is employed a small ivory disc as a rubber, which counteracts the friction of the needle on its axle.
7th. Of a fine concentric brass pulley on the axle of the needle, over which passes a fibre of unspun silk, carrying a small hook at each end, for the purpose of testing magnetic intensities by the employment of small weights, a series of which are furnished with the instrument.
8th. Of a pair of small cylindrical magnets fitted into brass tubes, and made to screw into the armature behind; when the verniers of the armature are placed to read off the same degree as the points of the needle, then the points of these magnets are exactly opposite thereto, and are employed in deflecting the needle from its true position, and thereby determining magnetic forces, and also of correcting the dip.
9th Of a thermometer, for registering temperature, when observations are made with the instrument.
Lastly. Of an extra needle, to be employed for correcting errors, arising from accident or otherwise, in the needle generally used. This is effected by taking the mean of several readings of the dip of this needle, with the face of the instrument towards the east, moving the horizontal limb through 180°, and taking the mean of several readings west, then of reversing the poles of the needle, and again taking the mean of several readings east and west, and lastly of taking the mean of these two results; if these correspond with the dip shown by the needle regularly employed, then the dip is correct. A mean of several readings will generally give the dip within a minute or two. The instrument may also be employed for measuring angles in azimuth and altitude, or be used as a theodolite. In using it as a magnetic variation, or declination instrument, the true meridian can be ascertained in the usual manner, by means of the telescope and the vertical and horizontal movements. The magnetic meridian can be exactly determined by reading off the azimuths when the needle is vertical, or rather when one of its poles, or points, is at 90° facing north as well as south, and the mean of the azimuths gives the magnetic meridian. These observations may be multiplied by turning the face of the needle towards the back of the instrument, and also by employing the other needle to repeat the observations. The method of different azimuths may also be employed ad libitum, reading off when the point of the needle is 1/2°, 1°, 2°, etc., on each side of 90° in succession. In all cases the rubbing of the point of the stud, or pin at the back, should be continued till the needle has settled in its place of rest. It should be gently done, especially at the last.
[Fox's dipping-needle deflector has now been employed by Ross, Stanley, Belcher, and others, for determining the variations of the dip, and intensity of magnetic force, for many years, with the most marked advantage; as it enables the observer to determine, with great accuracy even at sea, various points of interest in connection with the phenomena of terrestrial magnetism. The needle being brought to its true bearing at any spot on the earth's surface, a small balance is hung on the disc around the centre of suspension, and the weight required to bring the needle back to zero is the measure of the intensity of the magnetic force in action. For a detailed account, the Second Annual Report of the Royal Cornwall Polytechnic Society should be consulted.—R. H.]
Practical miner's theodolite, or improved dial and quadrant, used for underground and surface surveying, and constructed so as to answer all the purposes of the scientific and practical miner, while it may also be used as a surveyor's theodolite. This instrument is shown in the following cut.