352 



WRITINGS OF JOSEPH HENRY. 



[1855- 



fine wires : c 6 is a metallic foot, and h h slips of tin- foil 

 pasted on the inside and outside of the glass to discharge 

 the pith balls when the electricity is 

 so strong as to cause them to strike the 

 glass. To measure the electrical in- 

 tensity with this instrument the hook 

 a was removed, and its place supplied 

 with a pointed brass rod. The elec- 

 troscope was first brought in contact 



Fig. 13. Fig. 14. 



with the ground as exhibited in Fig. 13; then held vertically 

 as shown in Fig. 14, and gradually elevated until the leaves 

 began to diverge. Saussure found that the height to which 

 the instrument was required to be elevated before the leaves 

 showed signs of electricity varied at different times, and he 

 estimated the intensity of the electricity of the atmosphere 

 by the inverse ratio of this height. 



The explanation 

 of this will be read- 

 ily seen by a refer- 

 ence to Fig. 15, in 

 which C, D, repre- 

 sents a portion of 

 the surface of the 

 earth negatively 

 charged, and a 6 c, a 

 perpendicular con- 

 ductor terminated 

 above and below 

 by a bulb. In this 



+ 



Fig. 15. 



