86 



Alujttle 



Fig. 26 

 Fig. 26. Polarization of electrodes. 



If a piece of fresh animal tissue connects the pair of wire 

 electrodes, instead of the solution, the same polarization 

 occurs. Chemical chang-es occur where the wires touch the 

 tissue which can act in the reverse manner, and set up a small 

 current if the battery be removed and the electrodes connected 

 by a conductor. This acts as a source of fallacy in many 

 experiments and is of much importance when a very excitable 

 tissue, such as a nerve, is dealt with. The following experi- 

 ment will illustrate polarization. 



Arrange the apparatus as shown in fig. 26, open the key, 

 k2 and close the key, kl. The current is sent through the 

 nerve and will polarize it- There is no contraction of the 

 muscle while the current is passing. After one or two min- 

 utes, open kl, and then rapidly close and open k2, when con- 

 tractions will occur, which are due to the closing and opening 

 of the small current set up by the polarized electrodes. The 

 contractions diminish quickly in amount as the nerve becomes 

 depolarized. 



In order to avoid polarization effects, special forms of 

 electrodes may be used. These are known as unpolarizable 

 electrodes and usually consist of a glass tube containing a 

 saturated solution of zinc sulphate. The electrode end of the 

 tube is filled up with a pad of china clay or camel's hair brush, 

 upon which the nerve is laid ; the other end of the tube is 

 fitted with a binding post attached to a zinc wire which dips 

 into the zinc solution. The electrodes in the moist chamber 

 are an example of unpolarizable electrodes. 



