PHYSICS OF NINETEENTH CENT. ELECTRICITY. 591 



Pi 



FIG. 301. THE MIRROR GALVANOMETER. 



instruments. The magnetic needle is reduced to a mere fragment of a 

 steel watch-spring attached to the back of a circular mirror one-third 



FIG. 302. ARMSTRONG'S HYDRO-ELECTRIC MACHINE. 



of an inch in diameter, made of glass as thin as paper. The needle 

 is suspended by a single cocoon-fibre, in the midst of a coil of fine 

 copper wire. The light of a lamp D passing through a narrow slit in 



