4oS OPTICAL PROJECTION 



falls in the centre leg according to the direction of the 

 current. 



Or two open ends resembling a may be connected by a 

 horizontal tube about 1 mm. bore, and filled with mercury, 

 except that in the centre of the horizontal tube is introduced 

 2 or 3 mm. in length of the dilute acid. Behind the tube is 

 a glass divided scale. This represents for easy projection 

 Lippmann's capillary electrometer, being simply on a larger 

 scale the apparatus shown in fig. 127, p. 240. Very moderate 

 currents will readily cause motion of the acid index-spot upon 

 the screen. 



The beautiful experiments of Profs. A. W. Eeinold and 

 A. W. Eiicker, showing that an upward current retards, and 

 a 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 employ potash oleate, but 

 solution (c) on p. 826 will answer very well. 1 



244. Effects of the 

 Current. The decomposi- 

 tion of acidulated water is 

 easily projected by a simple 

 cell as shown in fig. 280. 

 Inverting a small test-tube 

 filled with fluid over each 

 wire, the approximate 

 volumes of the two gases 

 FIG. 230 ~~ ""* will be shown. By similar 



apparatus slightly modi- 

 fied as required, the decomposition of solutions of various salts 

 is projected in the same way. In this experiment, as in many 



1 See for details, Proc. Phys. Society of London, vl 857. It only need be 

 said here that the films are easiest managed as cylinders, formed between 

 metal rings or short cylinders, which form terminals for the current, the whole 

 being enclosed in a glass case to exclude dust and draught. 



