PHYSICAL FORCES 123 



jar's knob to the conducting piece of metal. A round 

 form is given to the knob because it is much more easy 

 to retain electricity in a body of that shape. In 

 projecting edges and points, the energy becomes so 

 concentrated and intense, that it cannot well be confined 

 in bodies so shaped. 



When a metallic circuit is complete, a current of elec- 

 tricity may readily be made to traverse it, but the 

 current ceases immediately the circuit is broken. Elec- 

 tricity is conducted along copper wire with extreme 

 rapidity, as before stated, and currents may be exceed- 

 ingly powerful. If the metallic circuit through which 

 such a current passes be of unequal capacity, then its 

 thinnest part may become, and be maintained at, a red 

 or even a white heat, reminding us of the accelerated 

 flow of a river where its banks approach each other and 

 the stream is narrowed. 



Currents of electricity have very wonderful and com- 

 plex effects to which it is impossible here to allude more 

 than distantly, the reader being referred, for all but 

 the very elements of the science, to special works on 

 Electro-dynamics and on the relations of electricity to 

 heat, or thermo-electricity. 



But the effects of currents of electricity and certain 

 modes of their generation, cannot be understood without 

 some elementary notions of magnetism and chemical 

 energy, so that what further remains for us to say about 

 electricity will be said when we come to speak of those 

 energies.* It may, however, be here briefly noted 

 that, whereas bodies in similar electrical conditions 

 repel, and in opposite electrical conditions attract, 

 each other, currents flowing in the same direction 



* See post, pp. 127, 133 and 134. 



