MODERN VIEWS WITH RESPECT TO ELECTRIC CURRENTS 665 



Thus we see that the theory of lightning rods, which appeared so sim- 

 ple in the time of Franklin, is to-day a very complicated one, and re- 

 quires for its solution a very complete knowledge of the dynamics of elec- 

 tric currents. In the light of our present knowledge the frequent fail- 

 ure of the old system of rods is no mystery, for I doubt if there are a 

 hundred buildings in the country properly protected from lightning. 

 With our modern advances, perfect protection might be guaranteed in all 

 cases, if expense were no object. 



So much for the rod itself, and now let us turn to other portions of 

 the electrical system, for we have seen that, in any case, the conductor is 

 only the core of a disturbance which extends to great distances on all 

 sides. Were the clouds, the earth and the streak of heated air called the 

 lightning flash all perfect conductors we could calculate the entire dis- 

 turbance. It might then consist of a series of stationary waves between 

 the two planes, extending indefinitely on all sides but with gradually de- 

 creasing amplitude as we pass away from the centre. The oscillations, 

 once set up, would go on forever, as there would be no poor conductors to 

 damp them. But when the clouds and the path of the lightning both 

 have very great resistance, the energy is very soon converted into heat 

 and the oscillations destroyed. I have given it as my opinion that this 

 is generally the case and that the oscillations seldom take place, but I 

 may be wrong, as there is little to guide me except guesswork. If they 

 take place, however, we have a ready explanation of what is sometimes 

 called a back stroke of lightning. That is, a man at the other end of 

 the cloud a mile or more distant from the lightning stroke sometimes re- 

 ceives a shock, or a new lightning flash may form at that point and kill 

 him. This may be caused, according to our present theory, by the 

 arrival of the waves of electrical disturbance which might themselves 

 cause a slight shock or even overturn the equilibrium then existing and 

 cause a new electric discharge. 



We have now considered the case of oscillations of electricity in a few 

 instances and can turn to that of steady currents. The closing of an 

 electric current sends ethereal waves throughout space, but after the 

 first shock the current flows steadily without producing any more waves. 

 However, the properties of the space around the wire have been per- 

 manently altered, as we have already seen. Let us now study these prop- 

 erties more in detail. I have before me a wire in which I can produce a 

 powerful current of electricity, and we have seen that the space around 

 it has been so altered that a delicately suspended magnetic needle can- 

 not remain quiet in all positions but stretches itself at right angles to 



