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MAGNETISM AND ELECTRICITY 40? apparatus may be made by taking a piece of ' quill' glass tube 4 inches long, and winding one half of it with two layers of insulated wire. Provide a soft iron wire 2J inches long, which slides easily in the tube. Fix the tube horizontally across the condensers, and place the iron core in the uncovered end, so that one end rather projects. Turning on the current, the core will be drawn into the coil, and the two ends will be seen projecting slightly at each end. It is easy to con- struct two small ' differential' coils in the same way. 242. Thermo-Electricity.—Seebeck's fundamental experi- ment is easily projected by simply placing it upon the vertical attachment. The current from two single wires about 6 inches long, of copper and German silver, simply twisted to- gether at one end and united by a drop of solder, can be easily shown by a decent projection galvanometer. Any experi- ments with the thermo-pile also afford illustrations. 243. Current and Capillarity.—Kiihne's beautiful experi- ment is easily projected, using a very shallow watch-glass or a plano-concave lens on the vertical attachment. The hollow is filled with dilute sulphuric acid containing a little chromic acid, and enough mercury to form an oblate spheroid half an inch across or more, introduced into the centre. A bright iron wire about 2 mm. diameter and 6 inches long is then introduced so as just to touch the edge FlG- 2 29 of the mercury, when the circumfer- ence of the spheroid is thrown into rhythmic vibration by the changes in surface-tension, set up by the intermittent contact and consequent intermittent current. Another simple apparatus is shown in fig. 229. The white portion a represents mercury in the glass tube, and the dark part b dilute sulphuric acid. On sending a current through the whole, it will be seen that the mercury rises or