Fig. 18.
Fig. 19.
The Base, to which all the other parts are fastened, is made of ¾ in. mahogany or teak, 6 in. by 4 in., shaped as shown at [Fig. 17], with a smooth surface and French polished. To this is attached the metallic base-plate, which may be cut out of sheet-iron, or sheet-brass (this latter is better, as iron disturbs the action of the magnet somewhat), and shaped as shown in [Fig. 18]; or it may be made of cast-iron, or cast in brass; or a substitute for it may be made in wrought-iron, or brass, as shown in [Fig. 19]. I present these various forms to suit the varied handicrafts of my readers; for instance, a worker in sheet metal may find it more convenient to
manufacture his bell out of the parts sketched in Figs. [17], [18], [20A], [21], [23], [24A], and [25]; but, on the other hand, a smith or engineer might prefer the improved form shown at [Fig. 31], and select the parts shown at Figs. [20A], [22], [19], choosing either to forge the horse-shoe magnet, [Fig. 20], or to turn up the two cores, as shown at [Fig. 21] (A), to screw into the metal base, [Fig. 21] B, or to be fastened by nuts, as shown at [Fig. 19]. The result will be the same in the end, if good workmanship is employed, and the proper care taken in fixing and adjusting the parts. A tin-plate worker may even cut his base-plate out of stout block tin, and get as good results as if the bell were made by an engineer. In some makes, the base-plate is cut or stamped out of thick sheet-iron, in the form shown by the dotted lines on [Fig. 18], and when thus made, the part A is turned up at right angles to form a bracket for the magnet cores, the opposite projection is cut off, and a turned brass pillar is inserted at B to hold the contact screw, or contact breaker ([§ 41]).
The Magnet may be formed as shown at [Fig. 20], or at [Fig. 20A]. Its essential parts are: 1st. Two soft iron cores (in some forms a single core is now employed); 2nd. An iron base, or yoke, to hold the cores together; 3rd. Two bobbins wound with wire. The old form of magnet is shown at [Fig. 20]. In this form the cores and yoke are made out of one piece of metal. A length of round Swedish iron is bent round in the shape of a horseshoe; this is rendered thoroughly soft by annealing, as explained further on. It is absolutely essential that the iron be very soft and well annealed, otherwise the iron cores retain a considerable amount of magnetism when the current is not passing, which makes the bell sluggish in action, and necessitates a higher battery power to make it work (see [§ 14]). Two bobbins of insulated wire are fitted on the cores, and the magnet is held in its place by a transverse strip of brass or iron secured by a wood screw passing between the two bobbins. The size of the iron, the wire, the bobbins, and the method of winding is the same as in the form next described, the only difference being that the length of the iron
core, before bending to the horse-shoe form, must be such as to allow of the two straight portions of the legs to be 2 in. in length, and stand 1⅜ apart when bent. We may now consider the construction of a magnet of the form shown at [Fig. 20A]. To make the cores of such a magnet, to ring a 2½ in. bell, get two 2 inch lengths of 5/16 in. best Swedish round iron, straighten them, smooth them in a lathe, and reduce ¼ in. of one end of each to 4/16 of an in., leaving a sharp shoulder, as shown at [Fig. 21] A. Next, get a 2-in. length of angle iron, drill in it two holes 1⅜ apart, of the exact diameter of the turned ends of the cores, and rivet these securely in their places; this may be done by fastening the cores or legs in a vice whilst they are being rivetted.