-1859] WRITINGS OP JOSEPH HENRY. 343 



other magnetizing spirals of iron wire, / and y, were ar- 

 ranged on opposite sides of the tube, the ends soldered to 

 the iron. When these two spirals were also furnished with 

 needles, and a discharge from a Leyden jar sent through the 

 apparatus, as if to pass along the wire, the needle inside of 

 the iron tube was found to exhibit no signs of magnetism, 

 while those on the outside presented strong polarity. This 

 result conclusively shows that notwithstanding the interior 

 copper wire of this compound conductor was composed of a 

 material which offered less resistance to the passage of the 

 charge than the iron of which the outer portion was formed, 

 yet when it arrived at the tin-foil covering of the cork, it 

 diverged to the surface of the tube, and still further diverged 

 into the iron wire forming the outer spirals. We must not 

 however conclude from this experiment that the electricity 

 actually passed on the outside of the tube. On the contrary, 

 we must infer from the following fact that it passes just 

 within the surface. If the iron be coated with a thin cover- 

 ing of sealing-wax, the latter will not be disturbed when 

 a moderate discharge is passed through it, though with 

 a large discharge in proportion to the conducting power of 

 the rod, the outward pressure may become so great as to 

 throw off the stratum of sealing-wax. This point is of some 

 importance in regard to the question of painting lightning- 

 rods. If the metal is of sufficient size to freely transmit an 

 ordinary discharge from the clouds, the condition of the ex- 

 terior surface can have but little effect, and we see no objec- 

 tion to coating it with black paint, the basis of which is car- 

 bon, a good conducting material. 



It is also to the same repulsive energy that we may at- 

 tribute the spreading of a discharge when it passes through 

 partial conductors, as in the case in which a spark from an 

 electrical machine is transmitted over a pane of glass on 

 which particles of iron filings are sparsely scattered. It is 

 probable that drops of rain and partially condensed vapor 

 in the atmosphere are in some cases connected with a simi- 

 lar appearance of discharge of electricity in the heavens. 



A much longer spark of electricity can be drawn through 



