STIMULI AND THEIR ACTIONS 



411 



current. Hence, an essential difference between the making and 

 the breaking induction-shocks is observed in the secondary coil, 

 since with the former, on account of the slow increase of the 

 primary current, the electric tension is more gradually equalised 

 than with the breaking shock, where the equalisation takes place 

 very suddenly. If, therefore, we wish to let a very sudden current 

 act upon a living object, we employ exclusively the breaking in- 

 duction-shock. The intensity of the induction-shocks may be 

 graduated by the distance that is allowed between the primary 

 •and the secondary coils. The intensity is less with greater 

 •distance, greater with smaller distance, and greatest when the 

 secondary coil, which is always made somewhat the larger, is 

 shoved completely over the primary. 



The sledge-induciorium of du Bois-Reymond, which is one of 

 the most essential of all pieces of physiological apparatus, is 

 constructed, in accordance with the above principles, for the pro- 



■■^ 



.yp<.:: 



h\G, 195. — Neef s or Wagner's hammer. 



duction of induced currents ; in it the secondary coil slides upon a 

 sledge-like track (Fig. 196). It is arranged also to produce single 

 induction-shocks rapidly and rhythmically. The contrivance that 

 makes this possible is the Neef or Wagner hammer (Fig. 195), 

 which is based upon the following principle. As is well known, 

 the galvanic current has the peculiarity of transforming into a 

 magnet a piece of soft iron, around which it flows, as long as the 

 circuit remains closed. If the current be broken, the magnetism 

 disappears. In Neef s hammer there is a brass column S, which 

 bears a straight spring. This spring, to the free end of which 

 is fastened a small hammer of soft iron, in its resting position 

 touches an adjustable screw I', which is in connection with a wire P; 

 the wire forms a coil about a soft iron rod which stands upright 

 under the hammer, and ends in a second small brass column. The 

 two brass columns bear screws to hold the conducting wires coming 

 from the element K If the current of the element be made, the 

 following happens : the current enters through the brass column S 



