358 GENERAL DIRECTIONS. 



them too wet, and especially do not let a bridge of fluid form 

 along the nerve between the two electrodes. 



Bring the wire from each electrode to the outer binding screw 

 on each side of a Du Bois Raymond's key (fig. 300). Bring 

 the wires from the battery to the inner screws of the same key. 

 Let the positive wire, the wire connected with the copper, 

 carbon, platinum, etc., of the battery be colored of some defi- 

 nite color, e. r/., red; let the wire fastened to the same side of 

 the key have the same color. The electrode connected with 

 this wire will be the positive electrode, or anode* Let the two 

 other wires connecting the zinc of the battery with the key, 

 and that side of the key with the other electrode which there- 

 fore becomes the negative electrode or kathode, be colored of 

 some other color, e. g., blue. 



When the key is down, the brass plate offers such little re- 

 sistance to the passage of the current, compared with that 

 offered by the nerve, etc., that the whole current will pass 

 through the bridge of the ke}% and none through the nerve. 



Consequently, opening the key is equivalent to throwing a 

 current into the nerve; shutting the key, to removing the cur- 

 rent from the nerve; during the whole time that the key is 

 open, the nerve, etc., is exposed to the action of the current. 



When the kathode (negative pole) is placed at a point on the 

 nerve nearer the muscle than the anode (positive pole), the 

 current is said to ^descending; when the anode is the nearer, 

 the current is said to be ascending. 



Single Induction Shock. Connect each wire from the 

 battery (Fig. 276 B), a key b intervening, with one of the two 

 screws on the top of the primary coil C. Connect the secondary 

 coil I) with the electrodes E E', a key a intervening. 



Whenever the key b is opened, and the current from the 

 battery allowed to pass from the primary coil, a current is in- 

 duced for the instant in the secondary coil ; another current 

 is similarly induced in the secondai\y coil when the same key 

 is shut; but in the interval there is no current produced in 

 the secondary coil provided that the current in the primary 

 coil be constant. 



If the key a is kept open while the key b is being opened or 

 shut, at each opening or shutting of b a single "induction 

 shock" is sent through the nerve. 



If a be kept open when b is opened, i. e., when the current is 

 allowed to pass into the primary coil (when the current is 

 made), but closed before b is closed again, a " making or closing 

 induction shock" only will be sent through the nerve. 



If the key a be kept closed while b is opened, and opened 

 before b is shut (and the current in the primary coil is broken), 

 a "breaking or opening induction shock" onl}- is passed through 

 the nerve. 



