20 Recent Advances in Electricity. 



but whatever it is, the metals, if they undergo it, do not oppose 

 elastic resistance to it ; hence the kind of energy which it 

 involves cannot be stored up in metals. 



I have now to explain how the theory is applied to electric 

 currents. Consider the case of an electrical current flowing 

 through a wire which joins the two terminals of a galvanic cell. 

 We must suppose each tube of force to stretch across in the first 

 instance from the positive to the negative plate of the battery, 

 and then to move along in such a manner that its ends or feet 

 slide along the wire until they meet at the middle of its length. 

 As its feet approach each other the tube shrinks in length, till it 

 vanishes altogether. This operation of sliding warms the wire, 

 the heat thus generated being the equivalent of the energy of 

 the tubes which have disappeared. In like manner, in the 

 discharge of a leyden jar, the force-tubes, which previously 

 stretched across from one coating to the other, slide up to the 

 place where the spark passes, and there shrink into nothing 

 in mid-air, thereby heating the air. When a telegraphic 

 current runs through a submarine cable, the tubes of force, 

 which extend radially from the copper core to the external 

 iron sheath through the intervening insulator, must be 

 regarded as travelling along through the insulator, so that 

 their two ends sHde along these two conductors. The use of 

 the copper core is to furnish a slide for one end of 

 the tubes of force, for without two slides, one for each 

 foot, they cannot travel. Here let me notice a remarkable 

 result which has been brought out in the supply of elec- 

 tricity to a district by means of alternating currents. It is 

 found that when the alternations are rapid it is a mistake 

 to employ solid wires or rods for the mains, and that much 

 less material will suffice if the mains are tubular. The 

 axial portion of a solid wire is practically inoperative when 

 the alternations are rapid ; and it is possible for alternations to 

 be so rapid that the currents are practically only skin deep. 

 From the point of view of modern theory, the action on the 

 wire begins at the surface, where the force-tubes slide upon it, 



