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 



