=418. Directions.= (A) Arrange the apparatus as in Exp. 164. Hold an iron strip (No. 76), a screw-driver, or other iron bar directly over and near the poles of the experimental electromagnet. Close the circuit at the key, then lift the magnets by the "armature," as the iron strip may be called, the circuit being kept closed for a few seconds. If your cell is good there should be no trouble in lifting the magnets by the armature. Open the circuit, and see whether the magnets drop. (B) Hold the magnets upside down directly over nails, tin boxes, iron filings, or other pieces of iron. Close the circuit, move the attracted iron to another place on the table, and open the circuit. Can this principle be used for practical purposes? _Note._--Some experiments illustrating practical uses of electromagnets will be given in a future chapter. =EXPERIMENT 173. Residual magnetism when magnetic circuit is closed.= =419. Directions.= (A) Arrange as in Exp. 164. You have already seen that each core retains some magnetism after the circuit is closed. Place the iron strip firmly across the poles, close the circuit for an instant, open the circuit, then see whether the armature still clings to the cores with some strength. The armature should fit well upon the cores for this experiment. (B) Again press the armature upon the cores, no current being used; then lift it as in (A). Compare the attraction with that found in (A). _=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. CHAPTER XXIV. THERMOELECTRICITY. [Illustration: Fig. 134.] =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.