This will at once rivet it in its place, and spread it sufficiently to make a good surface for contact. The screw must likewise be tipped with platinum, by having a small hole bored in the centre of its extremity, of the same diameter as the platinum wire, which must then be pushed in, and rivetted by hammering the end, and burring the sides of the screw. Whichever method be adopted, care must be taken that the platinum tip on the screw and the speck on the contact spring are adjusted so as to touch exactly in their centres. It will be hardly worth while for the amateur to cast or even turn up his own bells (which are generally of the class known as clock gongs), as these can now be procured so cheaply already nickelled ([see Fig. 28]). The bell must be adjusted on its pillar ([see Fig. 29A]), which is itself screwed into a hole in the base-plate, where it is held by a nut. The adjustment of the bell is effected by placing it over the shoulder of the pillar, and then clenching it down by screwing over it one or other of the nuts shown at [Fig. 29]. The bell should clear the base, and should be at such a height as to be struck on its edge by the hammer or clapper attached to the armature, Figs. [23] and [24]. We still need, to complete our bell, two binding screws, which may take either of the forms shown at [Fig. 27]; and an insulating washer, or collar, made of ebonite or boxwood, soaked in melted paraffin, to prevent the contact pillar ([Fig. 25]) making electrical
contact with the metal base. The best shape to be given to these washers is shown at [Fig. 30]. They consist in two thin circlets of wood or ebonite, that will just not meet when dropped, one on the one side, and one on the other of the hole through which the shank of the contact pillar passes when set up on the base-plate. If a wooden base be used below the metal base-plate, then only one washer, or collar, need be used—that is, the one above—since the screw of the pillar will pass into the wood, and this is not a conductor. If the metal base alone be used, both washers must be employed, and a small nut (not so large as the washer) used to tighten up and hold the pillar firm and immovable in its place opposite the contact spring.
Fig. 30.
Having now all the parts at hand, we can proceed to fit them together, which is done as follows:—The bell pillar, with its bell attached, is fastened by its shank
into the hole shown near B, [Fig. 17], where it is screwed up tight by the square nut shown at [Fig. 29 c]. In the same manner, we must fasten the contact pillar, or bracket, shown at [Fig. 24 A]. Whichever form be used, we must take great care that it be insulated from metallic contact with the metal base-plate by washers, as shown at [Fig. 30] (similar washers must be used for the two binding screws if the whole base-plate be made in metal). This being done, the metal frame, [Fig. 18], is put in position on the wooden base, as shown at [Fig. 17], and screwed down thereto by the screws indicated at s s s. The magnet may then be screwed down to the metal frame as shown. The small bracket of angle brass marked B, in Figs. [23] and [24], is next screwed into its place; that is, in such a position that the armature stands squarely facing the poles of the electro-magnet, but not quite touching them (say 1/16 of an inch for a 2½ in. bell). In setting up this and the contact pillar, the greatest care must be taken that the platinum tip of the contact screw, [Fig. 25], should touch lightly the centre of the platinum speck at the back of the spring, Figs. [23] and [24], shown full size at [Fig. 26].
The free ends of the helically coiled electro-magnet wires should now be inserted into short lengths of small indiarubber tubing (same as used for feeding bottles), the extremities being drawn through and 1 in. of the copper wire bared of its covering for the purpose of making good metallic contact with the connections. One of these ends is to be soldered, or otherwise metallically connected, to the angle brass carrying the armature, spring
and clapper, the other being similarly connected with the left-hand binding-screw, shown at [Fig. 17]. Another short length of wire (also enclosed in rubber tubing) must be arranged to connect the contact screw pillar [Fig. 17], with the right-hand binding-screw. When this has been done, we may proceed to test the working of the bell by connecting up the binding screws with the wires proceeding from a freshly-charged Leclanché cell. If all have been properly done, and the connections duly made, the armature should begin to vibrate at once, causing the "bob," or hammer, to strike the bell rapidly; that is, provided the platinum tipped screw touches the platinum speck on the contact spring. Should this not be the case, the screw must be turned until the platinum tip touches the platinum speck. The armature will now begin to vibrate. It may be that the clapper runs too near the bell, so that it gives a harsh, thuddy buzz instead of a clear, ringing sound; or, possibly, the clapper is "set" too far from the bell to strike it. In either case a little bending of the brass wire carrying the clapper (either from or towards the bell, as the case may dictate) will remedy the defect. It is also possible that the armature itself may have been set too near, or too far from the electro-magnet. In the latter case, the clapper will not vibrate strongly enough, in the former the vibration will be too short, and the clapper may even stick to the poles of the electros, especially if these have not been carefully annealed. A little bending of the spring, to or from the magnets, will remedy these deficiencies, unless the dis
tance be very much too great, in which case the bending of the spring would take the platinum tip out of the centre of the platinum speck.