MODERN THEORIES AS TO ELECTRICITY 289 



atom; we can imagine the atoms of chlorine and hydrogen in the body 

 of the liquid recombining with each other and their lines of force unit- 

 ing until they form a complete line long enough to stretch from the 

 zinc to the copper plate; and all without once making a line of force 

 without its end upon matter. We can further imagine the ends of this 

 line sliding along the copper and zinc plates to the conducting wires 

 and down their length, thus making an electric current and carrying 

 the energy of chemical action to a great distance. 



If the ends of the lines should slide along the wire without any 

 resistance, the wire would be a perfect conductor: but all substances 

 present some resistance, and in this case heat is generated. This we 

 always find where an electric current passes along a wire: as to the 

 exact nature of this resistance or the nature of metallic conduction in 

 general we know little, but I believe we are approaching the time when 

 we can at least imagine what happens in this most interesting case. 



Besides the heating due to the electric current, steadily flowing, we 

 must now account for the magnetic lines of force surrounding the cur- 

 rent and the magnetic induction of one current on the other. 



If the current is produced by the ends of the tubes of electrostatic 

 force moving along the wire, then we may imagine that the movement 

 of the lines of electrostatic force in space produces the lines of mag- 

 netic force in a direction at right angles to the motion and to the 

 direction of the lines of electrostatic force. At the same time we must 

 be careful not to assume too readily that one is the cause and the other 

 the effect : for we well know that a moving line of magnetic force (more 

 properly induction) produces, as Faraday and Maxwell have shown, an 

 electric force perpendicular to the magnetic line and to the direction of 

 motion. Neither line can move without being accompanied by the 

 other, and we can, for the moment, imagine either one as the cause of 

 the other. However, for steady currents, it is simpler to take the mov- 

 ing lines of electrostatic force as the cause and the magnetic lines as 

 the effect. 



We have now to consider what happens when we have to deal with 

 variable currents rather than steady ones. 



In this case we know from the calculations of the great Maxwell 

 and the demonstrations of Hertz that waves of electromagnetic disturb- 

 ance are given out. To produce these waves, however, very violent 

 disturbances are necessary. A fan waved gently in the air scarcely 

 produces the mildest sort of waves, while a bee, with comparatively 

 small wings moved quickly and vigorously, emits a loud sound. 

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