117.—Magnetic needles are generally made in the form of flat bars, which are balanced upon a standing point falling into a cup which forms the centre. When the greatest section of the bar is placed horizontally it is termed a broad needle, as shown Fig. 26. This may be made of the lozenge form shown, or be parallel throughout. When the greatest section is placed vertically it is termed an edge-bar needle, as shown Fig. 27. The north pointing end of the broad needle is commonly tempered dark blue, or has a deep cut across it, if the needle is left open. This is not necessary if it carries a ring. The edge-bar is generally used where it is required to read into a fixed circle of division, in which case its ends are brought to fine knife-edges.
118.—From the difficulty of reading a sharp point in bright metal against the black line of a divided circle, the author occasionally makes one point of the needle with a fine cut, sawn vertically for a short distance from its end, so as to form a kind of split which is afterwards closed, so that it presents the appearance of a fine black line of the same character as the divisions into which it reads. With this, as shown Fig. 28, the reading is found to be much more easy. The point is also more readily adjusted by grinding, as the end of the needle being broad, less care is necessary to avoid reducing it so much that it may leave the interior of the circle short where it reads into the divisions. This form of needle is not adapted to mining instruments, which have often to be read in an oblique direction.
Fig. 28.—Author's plan of needle reading.
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119.—In the Manufacture of the Needle it should be made of the finest cutler's cast steel, or, better still, of steel containing 3 per cent. of tungsten. If not left in a parallel strip as it is drawn or rolled, it should be brought as nearly to its form as possible by forging at a low heat. The steel should not be over-heated for hardening. It should be hardened in cold water or oil, and be tempered afterwards down to a very pale straw-colour—in fact, the temper colour should only just appear. Long needles may have the temper sufficiently lowered at the centre to set them approximately straight during the tempering; but the temper should not be lowered even in the centre below a pale blue, spring temper. After tempering, the setting and working up to balance is best done by grinding, and for the final adjustment, by stoning with Water-of-Ayr stone.
120.—Magnetisation of the Needle may be performed in many ways by means of a permanent magnet or an electro-magnet, or electrically by means of a solenoid. When the magnetism is induced from another magnet it is only important that the properly hardened needle should be regularly and equally magnetised over its surface by pressure upon it of the proper poles of the inducing magnet—that is, that the north pole of the magnet should induce magnetism in the southern half of the needle only; and the south pole in the northern half only.
121.—Method of Magnetisation by Single-touch.—This method is more generally applied to touching up needles than magnetising them at first. The northern pole of a strong permanent magnet is stroked down the southern end of the needle from its centre to its end three times on one side of the needle. The needle is then turned round, and the northern end is stroked down in like manner with the southern pole. The needle is then turned over, and the process is repeated on the other side. This may be done a second time and the edges of the needle be stroked down also.
122.—Method with both Poles.—In this process the needle is held down firmly with pegs on a board, and a strong horse-shoe magnet with rather close poles is laid on the bare needle without its cap, in a manner that both terminals press upon it. It is then drawn backwards and forwards from end to end of the needle several times, lifting the magnet finally from about the centre. The process is then repeated on the opposite side of the needle and its edges.