85 



of the two poles of the cell, the potential through the wire 

 will fall uniformly from the anode to the kathode. The 

 difference of potential between post and post 1 will be 

 practically one-tenth the electromotive force of the element. 

 Connect the battery (or two) through a key with the.rheocord, 

 at posts and 1. Arrange the electrodes from the "slider" 

 post to come in contact with the muscle. Close the key and 

 note the effect. Move the " slider " along the wires and note 

 the effect. 



An electric current can be graduated by changing the 

 number, arrangement and size of the cells, or by using a 

 rheocord to divide the current itself, the battery remaining 

 constant. 



317. Unipolar Excitation. Set up the battery and 

 inductorium to give single shoclis, and at first attach only 

 one wire to the secondary coil. Prepare a nerve-muscle pre- 

 paration and place it upon a dry glass plate, putting the 

 single wire from the secondary coil under the nerve. Open 

 and close the key in the primary circuit ; no contraction 

 occurs. Insert a second wire in the other binding post of the 

 secondary coil and attach its other end to a gas pipe thus 

 connecting with the earth. A contraction will now occur on 

 opening or closing the primary circuit. In the latter case, 

 the amount of current which passes through the earth and the 

 glass plate is suf&cient to stimulate the nerve. The short- 

 circuiting key in the secondary circuit is therefore used in 

 most experiments in order to avoid excitation of the nerve in 

 this way. 



318. Polarization of Electrodes. If a pair of clean 

 platinum wires be immersed in water, and a current sent 

 through them for a time, it is found that both of the plati- 

 mum terminals become covered with bubbles of gas. The 

 one in connection with the negative pole of the battery is 

 covered with hydrogen, and the other with oxygen. Upon 

 the removal of the battery and connection of the electrodes 

 with a galvanometer, a current will be demonstrated having 

 a reverse direction to that first induced. This condition at 

 the electrodes is known as polarization of electrodes. 



