_=420. Closed Magnetic Circuits.=_ It was seen in the study of the permanent horseshoe magnet, that the armature clung strongly to the magnet. The armature closed the magnetic circuit, the lines of force having almost no resistance. In the case of electromagnets the magnetic circuit becomes closed when the armature touches both poles at the same time. The armature clings strongly to the poles even after the current ceases to flow. As soon as the magnetic circuit is broken, however, but little residual magnetism remains. The armatures of electromagnets are usually arranged so that they can not quite touch the cores, to avoid this sticking.
=EXPERIMENT 174. To find whether electricity can be produced by heat.=
_Apparatus._ The home-made thermopile described in §421; astatic galvanoscope; connecting wires; candle or alcohol lamp.
=421. Home-made Thermopile.= (Fig. 134.) For this you need 3 hairpins, copper wire, a piece of wood about 3 in. long and 1 in. square on the ends, 2 pieces of tin, and some small nails.
Straighten the hairpins and scrape the coating off with sandpaper or a file. Scrape the insulation from 4 pieces of copper wire, each about 8 in. long. Twist the ends of the copper wire about the ends of the hairpins (Fig. 134), and then fasten the hairpins to the block. They may be held firmly by small nails which should be driven partly into the block and bent over. The hairpins at the right-hand side of the Fig. are shown to be near but not touching each other. This allows all to be heated at the same time.
The tin binding-posts may be nailed or screwed to the block, and if the bare copper wires 1 and 4 be placed under X and Y before they are screwed down they will be electrically connected. The ends of 1 and 4 may be held under the screw-heads. The block may be supported upon other blocks to raise it to the proper height, which will depend upon the length of the candle.
A thermopile in the form of a circle with several pairs of metals, can easily be made by fastening the hairpins to a piece of cardboard (Fig. 135) with a hole at the center. This may be supported by blocks, the heat being applied under the center.
=422. Directions.= (A) Arrange the apparatus as in Fig. 134. See that the astatic needle is properly adjusted, no magnets being near it.