334 



WRITINGS OF JOSEPH HENRY. 



[1855- 



taiice is still more surprisingly exhibited, by an arrangement 

 shown in Figure 10, which the writer adopted about the 

 same time during his electrical investigations at Princeton. 



The roof of the house 

 which he occupied in the 

 college campus was cov- 

 ered with tinned iron, and 

 this covering was there- 

 fore in the condition of 

 an insulated plate, on ac- 

 count of the imperfect 

 conduction of the wood 

 and brick-work which in- 

 tervened between it and 

 the ground. To one of 

 the lower edges of this 

 covering was soldered a 

 copper wire, which was 

 continued downward to 

 the first story, passed 

 through a gimlet-hole in 

 the window-frame into 

 the interior of the author's study, and then passed out of 

 the lower side of the same window, and thence into a well, 

 in which it terminated in a metallic plate below the surface 

 of the water. Within the study, the wire was cut and 

 the two ends thus formed were joined by a spiral of finer 

 wire a covered with silk thread. Into the axis of this spiral 

 a large sized sewing-needle d was inserted, the point having 

 been previously attached to a cork, which served as a handle 

 for removing it. With this arrangement, the needle was 

 found to become magnetic whenever a flash of lightning 

 was perceived, though it might be at the distance of several 

 miles. The intensity of magnetism and the direction of the 

 current were ascertained by presenting the end of the needle 

 to a small compass represented by c. In several instances 

 the inductive action took place at such a distance, that after 

 seeing the flash the needle was removed its magnetic con- 



Fia. 10. 



