240 STATE BOARD OF AGEICULTURE. 



the galvanic battery or from the electrical machine, be made to traverse wires 

 of different metals of equal length and diaineter in the same interval of tjme, 

 the rise of temperature in the wire is inversely proportional to its conducting 

 power, and therefore the better the conductor, the less doe? it emit heat." — 

 Millers' Physics, Yol. 1, p. 409. 



Eroni this it is evident that platinum, having only one-seventh the conduct- 

 ing poAver for electricity that copper has, would under the same circumstances be 

 sevens times as much heated l)y a fla^li of lightning as copper would, and a flash 

 which Avould heat a copper wire to its melting point (1,09C° Fah.) would heat a 

 platinum wire of the same size to 13,973° Fah. — a temperature greatly in excess 

 of the melting of platinum. Of the two metals tlierefore, platinum would be 

 more in danger of being melted by a flash of lightning than copper. For this 

 reason, and also because it is a cheaper metal, I prefer a point made of copper 

 with its surface gilded or nickel-plated to prevent rusting. 



You Avill also observe that these patented points which I show you are very 

 slender. They are objectionable on this account. In 1854 the Academy of 

 Sciences of France, the highest scientific authority on such matters, drew up 

 some instructions in regard to the construction of lightning-rod points. '"^The 

 instruction of 1854 returns to advise a copper point, seeing that copper, being a 

 much better conductor of electricity than platinum, it is less heated under the 

 action of this agent, and consequently runs less risk of being altered by fusion, 

 which frequently happens to platinum points. It is likewise necessary, with a 

 view to avoid this inconvenience, whatever may be the metal of which the jjoint 

 is made, not to make it too pointed. A conical point, under an angle of 30°, 

 is in this respect entirely suitable. It is true that we lose a part of the action 

 by induction, exercised over a storm-cloud ; but on the other hand we gain more 

 than Ave lose, by the incomparably greater resistance that is opposed to fusion 

 and to disintegration by lightning by a point AA'hich, instead of being drawn out 

 to a point, is pointed onlyAvithin the Ihnit indicated." — De laRiA'e's Electricity, 

 Vol. III., p. 1G2. 



A solid copper cone, two inches in vertical height and one inch in diameter 

 at the base, fulfills the conditions stated liy De la Rive. Such a point I now 

 shoAV you. It can be scrcAved onto the point of your iron bar used for a light- 

 ning-rod, and fulfills all the conditions required in such a point. You can get 

 these points of the Illinois Manufacturing Company of Adrian, Mich., of which 

 Henry 0. Hart is the agent. Single points can be obtained for $1, and Avhen 

 taken in quantity from 25 to 50, they can be got for 50 cents. It is not a pat- 

 ented article, and you do not have to pay a heavy royalty to the patentee AAdien 

 buying this point. 



IIOAV TO PUT UP A KOD. 



1st. For your rod, use round bar iron, not less than three-fourths of an inch 

 in diameter. This you can get of any dealer in liardAvare, and should not cost 

 you over ^4 for a hundred pounds. On the top of your rod have a shouldered 

 neck, Avith a scrcAV that Avill fit the ''female scrcAv" in the .base of your light- 

 ning-rod point. Get the blacksmith to bend one bar so as to give you a \-ertical 

 rod from the peak of your roof, and another bar to turn from the edge of your 

 roof, to reach the side of your building, to the ground. 



3d. The rod must be in metallic connection througli its Avhole lengtli. It is 

 not enough to hook or link the pieces togetlier. AVeld the pieces, or, more con- 

 veniently, scrcAV thenr together by means of AA'hat are called ''half -inch gas-pipe 



