306 FHYSIOLOGY OF MUSCLES AND NERVES. 



Occasionally it is desirable to remove this difference, and 

 to provide two inductive currents which flow and act nearly 

 in the same way. This may ba managed, if, instead of 

 closing and interrupting the current of the primary coil, an 

 additional closing wire offering small resistance is provided, 

 and the interruption is effected in this. If this additional 

 apparatus is present, only a very small part of the current 

 passes through the primary coil. The strength of this part is 

 indicated by J^ J ,. When the closing in the additional ap- 

 paratus is interrupted, the primary current slowly increases 

 in strengt^h from J , to J as shown by the dotted curve 5 ; 

 wdth this increase corresponds an inductive current in the 

 secondary coil, as represented by curve 6. If the closing of 

 the additional apparatus is once more effected, the current in 

 the primary coil sinks in strength from J to J •, but the so- 

 called extra current, that which originates in consequence of 

 the sinking in the primary coil, is now able, the coil being 

 closed, to take effect, and, as its direction is the same as that 

 of the main current, it retards the sinking of the latter, so 

 that this now takes place as indicated by curve 7 ; and with 

 this slow sinking of the main current corresponds an induc- 

 tive current in the secondary coil, such as is shown by 

 curve 8. 



Helmholtz made an alteration in du Bois-Reymond's 

 sliding inductive apparatus by means of which this ad- 

 ditional closing and opening is automatically accomplished. 

 He adapted Wagner's hammer for this purpose, as sliown in 

 fig. 74. The current of the apparatus A" passes through the 

 wire arranged between g and f to the primary coil c, from 

 this to the coils round the small electro-magnet 6, and from 

 the latter through the column ct, back to its original starting 

 point. The electro-magnet attracts the hammer A, in con- 

 sequence of which a small platinum plate fastened below 

 the German silver spring is brought into contact with the 

 platinum point of the screw/, thus completing a brief and 

 efficient additional closure g^f, a. The consequence of this 



