150 ELEMENTARY LESSONS ON [CHAP. in. 



foil on one side and with binoxide of manganese on the 

 other, piled upon one another, to the number of some 

 thousands, in a glass tube. Its internal resistance is 

 enormous, as the internal conductor is the moisture of 

 the paper, and this is slight ; but its electromotive-force 

 is very great, and a good dry pile will yield sparks. 

 Many years may elapse before the zinc is completely 

 oxidised or the manganese exhausted. In the Clarendon 

 Laboratory at Oxford there is a dry pile, the poles of 

 which are two metal bells : between them is hung a 

 small brass ball, which, by oscillating to and fro, slowly 

 discharges the electricity. It has now been continuously 

 ringing the bells for over forty years. 



183. Effect of Heat on Batteries. If a cell be 

 warmed it yields a stronger current than when cold. 

 This is chiefly due to the fact that the liquids conduct 

 better when warm, the internal resistance being thereby 

 reduced. A slight change is also observed in the E.M.F. 

 on heating ; thus the E.M.F. of a Daniell's cell is about 

 i J per cent higher when warmed to the temperature of 

 boiling water, while that of a bichromate battery falls off 

 nearly i 5 per cent under similar circumstances. 



LESSON XVI. Magnetic Actions of the Current. 



184. About the year 1802, Romagnesi, of Trente, 

 discovered that a current of electricity affects a magnet- 

 ised needle, and causes it to turn aside from its usual 

 position. The discovery, however, dropped into oblivion, 

 having never been published. A connection of some 

 kind between magnetism and electricity had long been 

 suspected. Lightning had been known to magnetise 

 knives and other objects of steel ; but almost all 

 attempts to imitate these effects by powerful charges of 

 electricity, or by sending currents of electricity through 



