70 ELEMENTARY LESSONS ON [CHAP. I. 



The difference of potential between zinc and carbon is 

 the same as that obtained by adding the successive 

 differences, or 1*09 volts. 1 Volta's observations may 

 therefore be stated in the following generalised form, 

 known as Volta's Law. The difference of potential 

 between any two metals is equal to the sum of the differ- 

 ences of potentials between the intervening metals in the 

 con tact-series. 



73. A difference of potential is also produced by the 

 contact of two dissimilar liqiiids with one another. 



A liquid and a metal in contact with one another 

 also exhibit a difference of potential. 



A hot metal placed in contact with a cold piece of 

 the same metal also produces a difference of potential, 

 electrical separation taking place across the surface of 

 contact. 



Lastly, it has been shown by Joseph Thomson that the 

 surface of contact between two non-conducting substances, 

 such as sealing-wax and glass, is the seat of a difference 

 of potentials. 



74. Magneto-electricity. Electricity, in the form 

 of currents flowing along in wires, can be obtained from 

 magnets by moving closed conducting circuits in their 

 neighbourhood. As this source of electricity yields 

 currents rather than statical charges of electricity, the 

 account of it is deferred to Lesson XXXVI. 



75. Summary. We have seen in the preceding 

 paragraphs how almost all conceivable agencies may 



1 For the definition of the voU> or unit of difference of potential, see Art. 

 33- 



