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Self-Induction. 



There is another phase of this problem, however. Between the years 1888 am;] 

 1892 Sir Oliver Lodge carried on an investigation of the phenomena of lightning, 

 by means of laboratory experiments, and to his surprise, as well as that of many 

 scientists and of the whole lightning rod fraternity, he found that an iron wire will 

 <3arry off a sudden rush of electricity better than a copper wire of the same size ! 

 Every sudden discharge or current of electricity induces an opposite current along 

 the same path. This is known as "self-induction." When the current ceases, 

 self-induction again takes place, this time opposing the drop in current just as it 

 formerly opposed the rise. Thus with an alternating circuit the self-induction 

 is high, and increases with the frequency, becoming enormous in the lightning 

 discharge, which oscillates about a million times per second. When a flash occurs 

 the resistance of the rod may frequently be insignificant compared with that offered 

 by self-induction. A steady current has no self-induction. While iron has greater 

 resistance to the steady current than copper, yet the self-induction of the iron is 

 less than that of copper in case of an electric spark or a flash of lightning. Basing 

 his judgment on this and related facts, Lodge stated that, in his opinion, the day 

 of copper lightning rods w^as done, although he added as a rider that in cities 

 and towns where coal is burned he thought the iron rods would not prove durable, 

 owing to the action of the fumes upon the zinc coating of the galvanized wire. We 

 are inclined to think that even in the country the question of durability is an 

 important one. Galvanizing is sometimes poorly done, and even if well done 

 corrosion takes place wherever the rods are cut or scratched. The same does not 

 apply to a copper wire. 



But it seems to us that in this judgment Lodge paid attention almost entirely 

 to one duty of the lightning rod, namely, to carry off the flash in case the building 

 is struck, for in one place he says : " I have at present no great faith in the effective 

 discharging power of a few points." By the data given at the commencement of 

 this bulletin we have seen that lightning rods have another and a greater duty to 

 perform, namely, the preventing of strokes from occurring. To prevent a stroke 

 there must be a gradual flow of electricity along the rod to the point and into the 

 air, or vice versa. As a matter of fact, it is easy to demonstrate that there is a steady 

 flow along the wire when the points on the toy buildings prevent sparks. The same 

 must be true in the real thunderstorm. Now, for steady current, copper rods have 

 a higher conducting power than iron ones, hence, for preventing strokes copper 

 rods are the better, while for carrying off strokes iron ones are the better. But 

 copper rods are made heavy enough to carry the " impulsive rush " and iron ones con- 

 ductive enough to carry the steady current, hence, durability is- the criterion, and 

 in this copper is indisputably superior to iron. 



In Michigan the Protected Mutual will not accept a risk on a building equipped 

 with iron rods, and the weight of copper rod on all of their buildings is at least 

 21/2 ounces per foot. The efficiency of their rods, as already mentioned, is 90.9 per 

 cent. The Patrons' Mutual, however, insures in its rodded class, whether the rods 

 are of iron or copper — and many of their buildings are rodded with iron, and yet in 

 eleven vears they have had to pay only three lightning claims on rodded buildings, 

 indicating about the same efficiency as with the Protected Company. So experience 

 would seom to support what has already been said, viz., that rods of any metal will 

 give good protection as long as they are in good repair and properly installed. So 

 that the relative value of the rods depends upon their respective durability. 



