Chap, iv.] HELMHOLTZ^ MODIFICATION. 49 



electro-magnet b. The weakening of the primary 

 current by the sudden closing of the short-circuit is, 

 however, effected gradually, because the circuit of the 

 primary coil is still complete, and the extra current 

 produced in the primary coil can make its effect felt. 

 The extra current is in the direction of the primary 

 current, and therefore tends to delay its weakening. 

 The consequent gradual weakening of the primary 

 current is represented by the dotted curve 3'. Pro- 

 duced by this is an induced current in the secondary 

 coil, indicated by the dotted curved line 4'. Thus 

 the original induction currents of the Du Bois coil 

 are represented by the continuous curves and lines 2 

 and 4 4, the disproportion between which is graphi- 

 cally shown ; while the induction currents produced 

 through Helmholtz' modification are represented by 

 the dotted curves 2'4', which are. as nearly as 

 possible, of the same intensity, and produce, therefore, 

 similar physiological effects. The sledge inductorium, 

 as now made, has the arrangement shown in Fig. 26, 

 and by proper adjustment of the screw f the modi- 

 fication is made available. By lowering the screwy 

 so that the spring cannot come into contact with it, 

 and by loosening the wire g', the modification can be 

 thrown out, and the original arrangement employed. 



It is to be noted that in the Helmholtz arrange- 

 ment the primary current is never quite interrupted. 

 It is only the increase and diminution in the strength 

 of the current round the primary coil that produces 

 the induced currents. 



E 7 



