THE STRING GALVANOMETER 33 



maximum sensitiveness is towards the yellow- 

 orange portion of the spectrum, but the inertia 

 appears to be unaffected by the wave-length, 

 though Korn has patented the method of using a 

 selected colour. Several experiments were made 

 in England screening the light with colour filters, 

 so that light of known wave-lengths 

 was used for illuminating the cells. 

 The results of a series of measure- 

 ments showed that there was no 

 advantage, the lag being the same 

 as when ordinary Nernst light was 

 employed. 



Before describing the actual re- 

 ceiving apparatus of Korn's selen- 

 ium machines it will be of interest 

 to see the construction of the gal- 

 vanometer, to which he has given 

 so much attention, and which is of 

 extreme sensitiveness, especially 

 when of the modified form as used in his later ' ' telauto- 

 graph " (see next chapter). The field magnets are 

 powerful, and the pole pieces measure about 

 5 X ii X 4 cm. These converge nearly to points 

 in the centres, as shown in Fig. 14. Thus they 

 nearly touch at AB, only sufficient room being left 

 for two fine silver wires to move freely between 

 them. The hole through the poles is* shown in the 

 diagram, and a sliding tube T fitted with a small 



P.T. D 



