238 STATE BOAED OF AGRICULTUEE, 



use for lightnmg-rods. Chemically pure copper stands next to silver as a con- 

 ductor, but this is also a costly metal compared M'ith iron, and besides, its con- 

 ducting power varies greatly, according to its purity. Iron is much inferior in 

 conducting power to copper. Chemically inire cojiper has more than six times 

 the conducting power of iron, but inferior commercial cojiper often has only 

 three times the conducting power of iron. Ordinary commercial copper may 

 safely be estimated to have four times the conducting power of iron. 



INFUENCE OF SIZE OF CONDUCTOR. 



The conducting power of a metallic bar of a given length and of uniform 

 size is jDroportional to its cross section. Thus an iron bar two inches square will 

 conduct electricity as well as a coi^per bar one inch square, because it has four 

 times the sectional area. By increasing the diameter of the bar of a poor con- 

 ductor, we may make it equal in conducting power to a bar of better conducting 

 material but of smaller size. While copper has four times the conducting power 

 of iron, it costs much more than four times as miich as iron. Iron is found in 

 the market in the form of long bars, well suited for use as lightning-rods, while 

 cojiper has to be specially manufactured for such use, Avherebyits cost is still 

 more increased, because a material which is manufactured for a special use costs 

 more than the same material manufactured for general use. Bar iron will cost 

 from 3| to 4 cents a pound, while bar copper will cost will cost about 50 cents. 

 With ordinary bar iron we may therefore obtain the same amount of conduction 

 at far less cash cost tlian we can with copper. 



Another point in favor of iron is that it is so difficult to melt, the melting 

 point of iron being more than 1,200° above the melting point of copper, A 

 lightning-rod should not melt or become red hot by any stroke of lightning 

 which may fall upon it. Arago of France, who directed especial attention to 

 this matter, from all the data he could collect on this subject, came to the fol- 

 lowing conclusions: " That a flash of lightning could melt completely, and 

 through its whole length, a chain of iron 130 feet long, of which the diameter 

 of the different links was not above a fourth of an inch ; that it might eifect the 

 fusion of a conical bar of copper nine feet long and one-third of an inch diam- 

 eter at the base ; that it could melt a leaden pipe of three inches diameter and 

 half an inch thick ; and finally, that lightning has never yet brought to red 

 heat a bar of iron of half an inch in diameter." — Smithsonian Keport for 1858, 

 p. 368. 



THE ROD. 



There lie on the table before me many specimens of lightning-rods, such as 

 are found in the country, and I tliink it would be difficult to crowd into so 

 small a space a corresponding exhibition of sham and scientific humbug. Here 

 is a rod made by twisting together three wires of tin. Tin is a comparatively 

 poor conductor of electricity, and it melts at so low a temjoerature that if a full- 

 grown flash of lightning should fall on such a conductor, there would be a spat- 

 tering shower of melted tin ! Here is a rod made by twisting together some fine 

 wires of copper with some coarser wires of zinced iron. The wag v;ho made this 

 lightning-rod probably thought that by making a conductor which would 

 resemble a galvanic battery the lightning would be so confused that it would 

 slink off into the ground and ask no questions ! Here is a rod made of copper 

 tubing, and another made of copper wire in the form of a roj^e. Here is a rod 

 made of iron, fluted and twisted into a spiral ; and here are others made in the 



