POLARISATION CURRENT 22!) 



passage of the current through the sheath). Similarly, below the 

 cathode there is a collection of electrons. If now the current is 

 switched off and the lower circuit (through a galvanometer) 

 switched on, it will be found that a potential difference has 

 developed, causing a current to flow through the galvanometer in 

 the opposite sense to the polarisation current. The previous 

 anode has become the cathode, 



i.e. the anode has been posi- /^ ^^^\\ Po'a'''sin<3 



tively polarised and the 

 cathode negatively polarised 



-? 



Polarisation model. The '^^W''^ Neqative polarisation. 



theory that the phenomenon j.-j^, 56._])iagram to sliow direction of tlic negative 



is due to a heaping up of l^lan^ation .uirent in a meduUated nerve. 



opposite charges in the sheath is svipportcd by a simple in- 

 organic model (Fig. 57). If for axon and sheath we substitute 

 a zinc wire and some cotton wool soaked in saline, we would 

 find that the cotton wool would collect positive charges under 

 the anode, negative charges under the cathode, and show a 

 negative polarisation in the same way as medullated nerve. The 

 substitution of zinc sulphate for the sodium chloride in the cotton 

 " sheath " prevents polarisation, indicating the probability that 

 the phenomenon is due (1) to the sheath, and (2) to some ionisable 

 substance therein. 



When an electric field is developed in water, each water molecule in the 

 field becomes polarised, i.e., oriented so that the positive ends (H) all point 

 in one direction, and, of course, the negative ends (OH) in the opposite 

 direction. The result of this arrangement is the formation of a large number 



Glass tube 



containing O-6/o Na Cj. 



Pt.wire 



c d d b e f 



Fig. 57. — Apparatus for iniitatinu the polarisation phenomena in medullated nerve. 



of tiny condensers in series stretching from one electrode to the other, thus 

 giving water a high value as a dielectric {q.v.). The polarity of the water 

 molecules, in other words, the presence of efficient condensers, may account 

 for the power water undoubtedly has of causing the dissociation of electrolytes 

 into ions {q.v.). If we measure the charge on a condenser formed by immersing 

 two metal plates in pure, air-free water, and get a value, we can then alter the 

 value by dissolving various salts in the water. The addition of .salts, ioni.sed 

 proteins, etc., increases the potential of the condenser. A charged condenser 

 can, of course, give up its charge (cf. Leyden jar). This is apparently what 

 happens when a current is passed through the medullated sheath of a nerve. 

 The discharge of the condensers gives rise to the negative polarisation current. 



