270 The Outline of Science 



certainly tells us to some extent what electricity is, and how it is 

 related to matter, but it leaves us with the usual difficulty about 

 fundamental realities. But we now know that electricity, like 

 matter, is atomic in structure ; a charge of electricity is made up 

 of a number of small units or charges of a definite, constant 

 amount. It has been suggested that the two kinds of electricity, 

 i.e. positive and negative, are right-handed and left-handed vor- 

 tices or whirlpools in ether, or rings in ether, but there are very 

 serious difficulties, and we leave this to the future. 



10 



What an Electric Current is 



The discovery of these two kinds of electricity has, how- 

 ever, enabled us to understand very fairly what goes on in elec- 

 trical phenomena. The outlying electrons, as we saw, may pass 

 from atom to atom, and this, on a large scale, is the meaning of 

 the electric current. In other words, we believe an electric cur- 

 rent to be a flow of electrons. Let us take, to begin with, a simple 

 electrical "cell," in which a feeble current is generated: such a 

 cell as there is in every house to serve its electric bells. 



In the original form this simple sort of "battery" consisted 

 of a plate of zinc and a plate of copper immersed in a chemical. 

 Long before anything was known about electrons it was known 

 that, if you put zinc and copper together, you produce a mild 

 current of electricity. We know now what this means. Zinc 

 is a metal the atoms of which are particularly disposed to part 

 with some of their outlying electrons. Why, we do not know; 

 but the fact is the basis of these small batteries. Electrons from 

 the atoms of zinc pass to the atoms of copper, and their passage 

 is a "current." Each atom gives up an electron to its neighbour. 

 It was further found long ago that if the zinc and copper were 

 immersed in certain chemicals, which slowly dissolve the zinc, 

 and the two metals were connected by a copper wire, the current 

 was stronger. In modern language, there is a brisker flow of 



