from it, the decrease being as the square of the distance that the current is removed from the wire. Magnets coiled with silk-covered wire admit also of better finish, but for most purposes cotton-covered wire will give satisfaction, especially if well paraffined. This wire must be wound on the bobbins, from end to end regularly, with the coils side by side, as a reel of cotton is wound. This may be done on a lathe, but a little practice will be necessary before the inexperienced hand can guide the wire in a regular manner. If, however, the spool of wire have a metal rod passed up its centre, and this be held in the hand at a distance of a foot or more from the bobbin on the lathe, the wire will almost guide itself on, providing the guiding hand be allowed to follow its course. With a little care, the wire for these little magnets may be wound entirely by hand. Before commencing to wind on the bobbins, just measure off 8 in. of the wire (not cutting it off) and coil this length around a pencil, to form a small coil or helix. The pencil may then be withdrawn from the helix thus formed, which serves to connect the wire with one of the points of contact. This free end is to be fastened outside the bobbin by a nick in the head; or the ⅛ in. length, before being formed into a helix, may be pushed through a small hole made on the head of the bobbin, so that 8 in. project outside the bobbin, which projecting piece may be coiled into a helix as above described. The wire should now be wound exactly as a reel of cotton is wound, in close coils from end to end, and then back again, until three layers of
wire have been laid on, so that the coiling finishes at the opposite end to that at which it began. To prevent this uncoiling, it should be fastened by tying down tightly with a turn or two of strong silk. The wire should now be cut from the hank, leaving about 2 in. of free wire projecting at the finishing end of each bobbin. In cases where many bobbins have to be wound, either for bells, for relays, or for indicator coils, a device similar to that illustrated at [Fig. 21 A] may be employed. This electric bobbin winder consists in a table which can be stood on a lathe or near any other driving wheel. Two carriers, C C, somewhat similar to the back centre and poppet head of a lathe, hollow inside, and furnished with a spring and sliding piston spindle, stand one at each end of this table. The sliding spindle of the one carries at its extremity a pulley, A, by means of which motion can be transmitted from the band of the driving wheel. The sliding spindles, B B, are fitted with recesses and screws, H H H H, by means of which the temporary wooden cores, or the permanent iron cores, of the bobbins can be held while the bobbins are being wound. The bobbin is placed as shown at D; a flat piece of metal, E, hinged at G, presses against the bobbin, owing to the spring F. The centre figure shows details of the carrier, C, in section. At the bottom is shown the spool of wire on a standard L. The wire passes from this spot between the two indiarubber rollers, M M, on to the bobbin D.
Fig. 21 A.
When the bobbins have been wound, they may be slipped over the magnet cores. They should fit pretty tightly; if they do not, a roll of paper may be put round
the magnet cores, to ensure their not slipping when the bell is at work. The helix ends of the bobbins should stand uppermost, as shown at [Fig. 22 A]. A short length of the lower free ends of wire (near the base or yoke) should now be bared of their covering, cleaned
with emery paper, twisted together tightly, as shown at [Fig. 22 B], soldered together, and any excess of wire cut off with a sharp pair of pliers. To prevent any chance electrical leakage between this bared portion of the wire and the iron, it should be carefully coated with a little melted guttapercha, or Prout's electric glue.
