150 



SCIENTIFIC AGRICULTURE. 



January, 1922. 



calculate the efficiency by the dollars 

 saved the average comes to 97.2 instead of 

 98.3, a difference of 3.9 percent, which is 

 a shade more than half of the 6.7. 



The reason why the rods as in general 

 use permit damage in 6.7 p. c. of the pos- 

 sible cases has been the subject of much 

 thought and research, even before' this 

 exact figure was known. From 1885 on- 

 ward to 1895, European scientists devoted 

 considerable time to the study of the cases 

 where damage occurred to rodded struc- 

 tures. Among the foremost of these ^vas 

 Sir Oliver Lodge, whose experiments and 



Left: Down cable on Tower of New Parliament 



Building. 

 Right: Closer view showing special bracket 

 fastening cable to Tower. 



writings on the subject, extended over 

 some five or six years. 



When the lightning rod was invented 

 the relative conductivity of different 

 metals was still unknown, and it was be- 

 lieved that any metal would be entirely 

 satisfactory, hence on account of its cheap- 

 ness and accessibility in suitable form, 

 iron was adopted by the early scientists 

 and erectors. But about 1820 Sir Hum- 

 phrey Davy, one of the earliest exper- 

 imenters on electrical conductivity, pub- 

 lished his comparative list showing tliat 

 copper is seven times as good a conductor 

 of steady current as is iron. By this time 

 there had been some failures of iron rods, 

 and by many the poor conductivity of iron 

 was at once seized upon as the cause. 



Others however still advocated iron and 

 then some copper rods failed, and so 

 there arose a controversy over material. 

 Avith men of science ranged on either side, 

 a controversy extending even to this day, 

 and which is being slowly decided in the 

 great laboratory of experience under the 

 guidance of new knowledge of the nature 

 of lightning and new electrical phenomena 

 in these later years, and consequently 

 additional laws of electricity applying. 



In the early years static electricity, tliat 

 is electricity existing in the form of sta- 

 tionary "charges'' either positive or neg- 

 ative on conducting bodies ,was the only 

 form knoMTi. Static induction, of course, 

 was kno-SATi, that is the production of a 

 new charge by the influence of one al- 

 j'cady in existence, the induced being of 

 <)p]iosite kind to the original. 



This accounts for the accumulation 

 of a charge on the earth directly be- 

 neath the thunder cloud. It was also 

 realized that when two charges rush to- 

 gether, a "current" is set up in the inter- 

 vening path whether it be of air or metal, 

 a current identical in properties with that 

 later produced by batteries and genera- 

 tors. Of all these properties the greatest 

 is current induction, discovered by Para- 

 day in 1831. If two wires lie side by side 

 and a current is sent through one, a mo- 

 mentary current is induced in the other in 

 opposite direction to the original, and 

 when the original is stopped, another mo- 

 mentary induction occurs in the secondary 

 wire, this being in opposite direction to 

 the first induction. If the secondary is a 

 closed circuit the impulses %vill travel com- 

 pletely round it, and if not they bank up. 

 first at one end of the wire and then at the 

 other producing a condition similar to a 

 static charge and favourable to a spark 

 and inviting side flash. Self-induction also 

 comes into play — when a current is first 

 forcing its way through a wire there is a 

 back induction in that same wire and when 

 the current is dying out another self in- 

 duction, this being in the same direction 

 as the original current. Static induction, 

 current induction iji neighboring conduc- 

 tors, and self-induction in the main con- 

 ductors, as well as the conductivity and 

 durability of the latter, must all be taken 

 into consideration when determining the 



