Chap. IV.] 



EXTRA CURRENT. 



47 



the more marked. In order to diminish the difference 

 between the two, Professor Helmholtz, of Berlin, has 

 devised a modification of the arrangement of the 

 coil. 



Modification of the Dn Bois iiidiictoriiuii. 

 This is shown in Fig. 26. It consists of an additional 

 pillar , at the foot of which is a binding screw k. 

 The top of the pillar has a screw with a fine platinum 

 point f'j which can be raised or lowered, so as to 

 touch or be removed from the spring, above it. An 

 additional binding - screw 

 g is put on the first pillar, 

 and a wire carried from 

 it to f. Let E be the 

 element, and let its posi- 

 tive electrode be carried to 

 screw d of the first pillar, 

 and its negative electrode 

 to the binding screw k on 

 pillar a. The course of 



fi , . ,, Fig. 26. The Helmholtz Modifi- 



bne Current IS as lOliOWS : cation of the Sledge Inductor. 



From E to the first pillar, 



up that pillar, out from g by the wire g', to binding 

 screw f, and so round the primary coil c, and then on 

 to the electro-magnetic pillars b. From b it passes, by 

 a wire laid in the wood of the instrument, to pillar a, 

 and from it through the binding screw k, to gain the 

 negative electrode, and so back to E. Now when the 

 current is passing through the electro-magnets 6, they 

 become magnetised, and attract the hammer-head h. 

 This pulls the spring down, and causes it to make 

 contact with the point of the screw f, which is 

 adjusted at the proper height for this purpose. As 

 soon as f is touched by the spring, a second pathway 

 is opened to the current, viz. from E to the first pillar, 

 up that pillar, and along the spring toy, by means of 

 /', down pillar a, and back to the battery E. By this 



