cases, and is represented in perspective in Fig. 228, and in symbolical sectional elevation in Fig. 229. The instrument consists of a hard permanent steel magnet, NS, fixed at the lower end to a highly permeable cast-steel pole-piece, P, of the form of a cup, and at the other end to a flat circular disc, D, of similar material. A circular gap, g, is thus formed between DP and P, just wide enough to allow a brass tube to slip freely through. A coil of fine wire is wound on the tube, which in slipping through the gap cuts the lines of force flowing across, and gives an electro-magnetic impulse to any circuit connected to the coil terminals fixed 9 D N P S P FIG. 229. to the top of the tube. Owing to the pole-pieces forming a nearly closed magnetic circuit, the magnet is quite permanent, and the total lines of force through the gap are constant. This number is usually about 20,000, so that the magneto-inductor is much more convenient than earth coils, which usually are relied on. In Fig. 228 one tube is just shown in position for slipping through the gap, to do which lightly span the milled head on its top and turn slightly, when the tube will drop by its own weight. A spare tube is shown on the left having a different number of turns, which is often very convenient. Earth Coil or Inductor. A form of ordinary earth inductor or coil is shown in Fig. 230. It consists of a hollow wooden ring wound with a large number of turns of insulated copper wire, the ends of which are connected to the two spindles which support the coil. These spindles work in bearings carried on the wooden framework, composed of two sides held rigidly together by a back. The two terminals of the inductor shown on the top of this back are connected to the two bearings respectively, and thence to the coil. The advantage of this construction is that the coil will rotate freely, no matter which side the inductor rests on. When resting on its back, or up against a wall, the spring stop seen is pushed out against the spring, and stops the coil rotating. This arrangement, however, is not to be recommended, as it only ends in damaging the bearings of the coil, thus producing bad contact with the coil. Fig. 231 shows a model of a two-pole dynamo made for the purpose of roughly investigating the distribution of the magnetic field under various conditions. It consists of an iron casting forming the two pole-pieces at the top, the limbs at the sides, and yoke at the bottom. The limbs are each wound with a coil connected to the centre and one of the outside terminals seen on the top, and hence either coil can be used separately to magnetize or both together by connecting to the two outside terminals. Four iron rings are provided of the same axial thickness, but having different external and internal diameters. These are to represent unwound armature cores of both the ring and drum types, and the effect on the flow of the lines of force between the polepieces by using them will, to a certain extent, show the effect of both the length of air-gap and sectional area of iron in the core on the magnetic distribution generally. The whole is fixed and let into a baseboard, so that a sheet of thin cardboard will lie flat on the top. The rings are held concentrically with the pole-pieces by slipping over a central pin and wooden rings when necessary. The general distribution of the magnetic field can then be found by sprinkling iron filings on the sheet of cardboard laid on the top of this representative machine. |