BY DR. MICHAEL FOSTER. 359 



In determining the direction of the induction shock, it must 

 be remembered that, at making the current in the primary coil, 

 the current induced in the secondary coil is opposite in direc- 

 tion to that of the primary, but that on breaking it, it is in the 

 same direction. 



Interrupted Current. For ordinary tetanizing purposes, 

 the magnetic interrupter of Du Bms Re3'mond's apparatus is 

 used (see Fig. 293). Connect the end of the positive wire of 

 the batteiy with the brass column gr, and the negative wire 

 with a ; the current then enters by (7, passes along the German 

 silver spring, which when not in action is in contact, by a little 

 plate of platinum soldered on to its upper surface, with the 

 platinum point of the screw/. From /the current passes 

 through the brass block e, with its binding screw d, to the pri- 

 mary coil c; after traversing it, it reaches the electro-magnet 

 6, and then returns to the battery through the binding screw a. 

 The anchor h is supported over the electro-magnet by the end 

 of the German silver spring; the moment that the current 

 passes through 6, the anchor with the spring is drawn down 

 so as to break the current at /. Thereupon, the magnet 

 ceasing to act, allows the spring to return to its former posi- 

 tion. By sliding the secondary coil to a greater or less dis- 

 tance from the primary, the strength of the induced current 

 can be varied at will. 



When it is specially important to avoid unipolar action, the 

 apparatus must be modified in the manner recommended by 

 Helmholtz. With this view, connect the column g with the 

 binding screw d by a side wire also marked g, and heighten 

 the tip of the column a by means of the milled rim. This 

 arrangement is shown in Fig. 294. The current enters as be- 

 fore, but in its course to the primary coil it passes partly 

 through /and partly through the side wire g. When the an- 

 chor is drawn down, as seen in the figure, the spring rests 

 upon the summit of a, so that the current passes directly back, 

 as indicated by the arrow, to the battery. The moment this 

 is the case, the current through the electro-magnet becomes so 

 feeble that it is insufficient to keep down the anchor, the spring 

 rising again comes into contact with/, and so on. The modi- 

 fication of effect is as follows : 1. The induced currents are 

 weaker, for the variations of the strength of the current are 

 less. 2. The intensity of the opening induction current, which 

 in the ordinary arrangement is much greater than that of the 

 closing, is reduced so that the two are nearly equal. 



If it is desired to allow the current from the battery to tra- 

 verse the primary coil without passing the interrupter, so as, 

 e. g., to use the apparatus for producing single opening or 

 closing induction shocks, connect the positive wire of the bat- 

 tery with e, and the negative, as before, with a. 



