MISCELLANEOUS PHENOMENA. 



321 



column in a capillary tube is used as an indicator of electromotive 

 force. 



25. Electric osmosis. * A U-tube AJB, Fig. 24, is filled 

 with water and provided with two platinum electrodes, and an 

 electric current is sent thrbugh the cell in the direction of the 

 arrows. The bend of the tube is filled with fine sand. Under 

 these conditions the water is 



found to rise in the arm B 



and fall in the arm A, or, 



in other words, the current 



causes the water to diffuse 



through the sand from A to 



B. This forced diffusion of 



a liquid through a porous 



diaphragm is called electric W$V J^Mi^-fine sand 



osmosis. It was discovered 



in 1807 by Reuss. This 



effect is greatly reduced 



when a good conducting liquid, such as an acid or salt solution, 



is used instead of water. In 1835 Becquerel discovered that 



fine particles of clay or other material suspended in water are 



caused to travel in one direction or the other when an electric 



current is sent through the water. 



26. The change of electrical resistance of selenium by illumi- 

 nation, f Willoughby Smith, in 1873, discovered that the elec- 

 trical resistance of metallic selenium is reduced to one half or one 

 third of its normal value when the selenium is exposed to brilliant 

 sunlight. 



27. Atmospheric electricity. It was shown by Benjamin 

 Franklin about 1760 that the lightning discharge is identical in 

 its nature to the ordinary electric spark. Very little was learned 

 after Franklin's time concerning the cause of atmospheric elec- 



*See Wiedemann, Die Lehre von der Elektridtdt, Vol. II, pages 166-195. 

 f See Wiedemann, Die Lehre von der Elektricitat, Vol. I, pages 547-55 1 - 



22 



Fig. 24. 



