. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. solder, can be easilyshown by a decent projection galvanometer. Any experi-ments with the thermo-pile also afford illustrations. 243. Current and Capillarity.—Kuhnes beautiful experi-ment is easily projected, using a very shallow watch-glass ora plano-concave lens on the verticalattachment. The hollow is filled withdilute sulphuric acid containing a littlechromic acid, and enough mercury toform an oblate spheroid half an inchacross or more, introduced into thecentre. A bright iron wire about
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. solder, can be easilyshown by a decent projection galvanometer. Any experi-ments with the thermo-pile also afford illustrations. 243. Current and Capillarity.—Kuhnes beautiful experi-ment is easily projected, using a very shallow watch-glass ora plano-concave lens on the verticalattachment. The hollow is filled withdilute sulphuric acid containing a littlechromic acid, and enough mercury toform an oblate spheroid half an inchacross or more, introduced into thecentre. A bright iron wire about 2 and 6 inches long is thenintroduced so as just to touch the edgeof the mercury, when the circumfer-ence of the spheroid is thrown into rhythmic vibration by thechanges in surface-tension, set up by the intermittent contactand consequent intermittent current. Another simple apparatus is shown in fig. 229. The whiteportion a, represents mercury in the glass tube, and thedark part b dilute sulphuric acid. On sending a currentthrough the whole, it will be seen that the mercury rises or. Fig. 229 OPTICAL PROJECTION falls in the centre leg according to the direction of thecurrent. Or two open ends resembling a may be connected by ahorizontal tube about 1 mm. bore, and filled with mercury,except that in the centre of the horizontal tube is introduced2 or 3 mm. in length of the dilute acid. Behind the tube isa glass divided scale. This represents for easy projectionLippmanns capillary electrometer, being simply on a largerscale the apparatus shown in fig. 127, p. 240. Very moderatecurrents will readily cause motion of the acid index-spot uponthe screen. The beautiful experiments of Profs. A. W. Reinold andA. W. Eiicker, showing that an upward current retards, anda downward current hastens the thinning of a liquid film,are projected in exactly the same way as an ordinary soap-film, described in Chapter XX. The usual liquid, however,thins too slowly. It is usual to emplo
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