544 



NERVE. 



\\ 



two parts of the extrapolar anodic region are connected with a galvano- 

 meter, a derivation of the surface effect will pass through it from the 

 contact nearer the anode to the more distant one, i.e., the proximal 

 anodic extrapolar contact will be galvanometrically positive to the 

 distal one, and a current will thus pass in the core from the latter 

 to the former. Similarly, the proximal cathodic extrapolar contact 

 is galvanometrically negative to a distal one. If the direction of 

 the polarising current in the axis cylinder or internal core has been 

 such as to be represented thus ^, then, on its cessation, the intra- 

 polar polarisation effect in this will be < ; the anodic extrapolar 



effect will be ^, and the cathodic X as shown in Fig. 282, 1. 



As a matter of fact, the above results are complicated when both 

 anode and cathode are present by bipolar electrolytic changes, anodic 

 and cathodic, occurring in close proximity to one another. The effect 

 of the changes in the neighbourhood of any one pole, " idiopolar," can 

 be thus disturbed by the spread of changes from the other pole. In 

 order to simplify the matter, a model may be made so as to be unipolar, 

 one contact only being in connection with the moist envelope, and the 



other being attached 

 to the projecting end 

 of the central metal- 

 lic core. 



Under these con- 

 ditions the idiopolar 

 effects only are,- seen, 

 and are always those 

 described ; i.e., on 

 the cessation of the 

 polarising current, 

 currents flow 

 through the core 

 towards the previous 

 anode and away 

 from the cathode. 

 With bipolar con- 

 tacts, the bipolar polarisation effect may swamp the idiopolar. In 

 the intrapolar region the bipolar polarisation is such that the anodic 

 region of the envelope is galvanometrically positive to the cathodic ; 

 but if the polarisation has spread along the moist envelope into 

 extrapolar regions, then such region on the anodic side will be also 

 galvanometrically positive to the cathodic, or to all points nearer 

 this cathodic portion. A current will thus flow in the envelope 

 from a distal point of the anodic extrapolar region towards a point 

 nearer the cathodic one, returning through the core the opposite way. 

 Hence, if the direction of the polarisation current in the core is, 



as before, represented by ^, although the intrapolar polarisation 



after effect is as before ^ , the anodic extrapolar effect will be ^ . 



A similar state of affairs may exist in the cathodic extrapolar region, 



giving a cathodic extrapolar current directed ^ . It will be observed 



that these bipolar effects are in the opposite direction to the idiopolar 

 effects just described (Fig. 282, II). 



By the use of very thin envelopes, the resistance offered by the 

 liquid envelope to bipolar polarisation effects is greatly increased, 



I 



JL 



FIG. 282. I. Extrapolar polarisation currents on opening a 

 polarising current led through model, with thin en- 

 velope from A to C (idiopolar effects). II. Extrapolar 

 polarisation currents in model, with large envelope and 

 tine core (bipolar effects). 



