ELECTRO-PHYSIOLOGY. 581 



emptied, rinsed in tap water, cleaned, and placed in several hundred 

 cubic centimeters of normal saline until wanted. If the boot is kept 

 saturated with normal saline, the electrodes will remain non-polar- 

 izable. 



Detector, or Galvanoscope, or Current Indicator. Use a vertical 

 galvanoscope, in which the magnetic needle is so loaded as to rest 

 in a vertical position. It consists of a magnetized needle, sur- 

 mounted by a coil of wire. It indicates the passage and direction 

 of a current. It really is a little galvanometer. Now, connect the 

 wires from the positive (+) and negative ( ) poles of a battery with 

 the binding screws., and note, when the circuit is closed, the needle 

 deviates from its vertical position. 



Fig. 217. Pohl's Commutator. (LAHOUSSE.) 



IKeys. When we wish to make or break a current by hand we 

 keys. DuBois key consists of two metal blocks, each carrying 

 i binding-screws fitted on a base of hard rubber, which acts as an 

 insulator. These two blocks of metal are connected by a metal 

 cross-bar which thus closes the key. It is employed in two ways. 

 In one, it breaks the current going from the cell to the nerve ; when 

 the key is closed the current is made, when the key is open the cur- 

 rent is broken. In the other way, the current from the cell passes 

 through the key when it is closed and then it is a short-circuiting 

 key, because the current going through the electrodes from the short- 

 circuiting key to the nerve meets here a body (the nerve) which 

 opposes a great resistance to the passage of the electrical current, 

 and, as electricity always takes the easiest way home, it goes through 



