36 



[Report of the Electrical Conference at Philadelphia in November, 1884, pp. 172-17-t; 



Washington, 1886] 



As this is an important question, especially in some of the Western 

 States, I will say a few words. 



In order to protect buildings from lightning we must have a space 

 into which the lightning cannot come, and have the house situated in 

 that space. What sort of a space do we know in electrical science into 

 which electricity cannot enter from the outside ? It is a closed space 

 I mean a space inclosed by a very good conducting body. All the light- 

 ning in the world might play around a hollow copper globe and it would 

 not affect in the slightest degree anything inside the globe; but the 

 the walls of the vessel need not be solid metal. Of course, if solid, it 

 is all the better ; but if it is made of a net-work of very good conducting 

 material it would protect the inside from lightning strokes. A spark 

 striking on one side of such wire cage would find it easier to go around 

 through the wire of the cage to the other side than it would to go 

 through the centre. This is Maxwell's idea, with reference to protec- 

 tion of houses from lightning, viz., to enclose the house in a rough cage 

 of conducting material. Suppose, for instance, this box is the house, 

 and suppose we start from the roof and run a rod diagonally to each 

 corner and thence down to the earth. We thus make a rough cage. 

 Of course there are openings on the sides; and if we wished to make a 

 better protection we could put rods down the sides wherever we wished. 

 Now, there is ground underneath the house, and the lightning might, 

 by jumping across the centre, find a good conductor through the middle 

 of the house and go down to the earth in that way. How do we prevent 

 that? By running the lightning-rods clear across underneath the 

 house. Then the lightning would find it easier to go around the house 

 than to jump across, even if there were a good conductor through the 

 middle. A house inclosed in a cage of that sort would be perfectly 

 protected, even if it were a powder magazine, or anything of that sort. 

 Of course, in the case of petroleum storage reservoirs, where fumes are 

 given off, there would be danger then, as the stroke might ignite the 



