ELECTRICAL PHENOMENA. 



99 



For purposes of this kind the string-galvanometer or the instrument known 

 as the capillary electrometer is employed. 



The String-galvanometer. In this instrument a very delicate thread 

 of silvered quartz or of platinum is stretched between the poles of a strong 

 magnet, as is represented in the diagrams given in Figs. 42 and 43. The 



b 



Fig. 42. One form of the string-galvanometer : E, The electromagnet; 6, the projection 

 microscope; F, a screw for varying the tension of the thread. (Edelmann's Catalogue.) 



Fig. 43. Schema to show the relation of the thread to the magnets in the string- 

 galvanometer : AA, The delicate thread of silvered quartz or of platinum, stretched between 

 the polar pieces (PP) of an electromagnet. When a current passes through AA, the thread 

 shows a movement. The ends of the magnets are pierced by holes, seen in PI, through 

 which the movements of the thread may be watched oy means of a microscope or be pro- 

 jected upon a photographic plate. (After Einthoven.) 



metal poles of the magnet are pierced by holes, so that the thread may be 

 illuminated by an electric light (arc light) from one side, and on the other 

 the shadow of the thread may be thrown upon a screen after being magnified 

 by a microscope (see Fig. 42). With this arrangement the thread shows a 



