STRING GALVANOMETER 



297 



passes in the direction of the arrow, from top to bottom of the 

 diagram, the fibre will bend outwards, i.e. in the direction of the 

 arrow a. Reversal of the direction of the current of course causes 

 reversal of the movement of the fibre. The excursions of the 

 string can be observed by means of the reading microscope AE, 

 which passes through a hole in the magnet, or records may be 

 made by placing an arc lamp at G, concentrating the light on the 



c s i 



613 



FIG. 64. Diagram of the essential parts of the string galvanometer. N and S 

 are the poles of a powerful electromagnet, between which is stretched the fibre C. 



fibre by a lens F and throwing the shadow on to a moving photo- 

 sensitive surface. Fig. 65 shows diagrammatically the arrange- 

 ments of galvanometer and accessories for photographing the 

 fibre movements. The distances are given in millimetres. 



The optical mechanism for producing the electrocardiograms 

 needs some mention. The camera is a light-tight box fitted with 

 a cylindrical lens and an arrangement whereby a sensitive photo- 

 graphic plate or film (or bromide paper) is made to travel at a 

 uniform speed past the narrow lens. The field of the objective 

 is projected by an eyepiece on to the lens which focuses it as a 

 spot of light on the part of the sensitive surface exposed by the 

 slit. The shadow of the fibre appears as a dark spot in this band 

 of light. Thus if the plate or paper be moved downwards normal 

 to the cylindrical lens, the whole surface will be exposed to the 

 action of the light except that portion protected by the shadow 



