222 ELECTRO-PHYSIOLOGY CHAP. 



tion of a neutral body, or better, of a conductor containing the 

 opposite charge, from the other end of the coil ( = induction}. 



When, as in the above experiment from Tiegel, one pole of 

 the secondary coil is led off to earth, there must at the moment 

 of induction be a certain (positive or negative) charge (potential) 

 at the metal plate connected with the other pole, which must be 

 twice as great as it would be if the other pole were not led off. 

 The effect of induction upon the second insulated metal plate, 

 parallel to the first, is to set up a potential of the opposite sign 

 which varies with the distance. At the next moment, the negative 

 electricity (let us say) of the charged pole flows through the 

 secondary coil to the other pole, and the positive electricity of 

 the plate charged by induction flows through the nerve-muscle 

 preparation to earth, whereby excitation is produced. 



The unipolar effects that occasionally appear (as was again 

 pointed out by du Bois-Eeymond) with incomplete closure of the 

 circuit are of great practical importance in all electrical experi- 

 ments with induced currents. If the nerve of a frog's leg is laid 

 across two electrodes connected with the poles of a secondary 

 coil, so as to close the induction circuit, a ligature being then 

 applied to the myopolar tract, tetanus may still be observed in 

 the isolated leg, on making the lead-off from it at a certain 

 distance of coil. It is evident that the same would occur if the 

 nerve were cut away above the crushed and no longer conducting 

 point, and replaced by any moist conductor. Here, as in all the 

 previous experiments with an open circuit, the direction of the 

 unipolar passage of the current makes itself apparent, in accordance 

 with the law of contraction. Excitation occurs only with charges 

 in which positive electricity leaves, or negative electricity enters, 

 the nerve. These unipolar effects may obviously be very dis- 

 turbing, and are indeed productive of fallacies in vivisection, and 

 also in experiments with the galvanometer, if not avoided by clue 

 precautions. Hering (61) has pointed out that in experiments 

 such as the investigation of the negative variation of nerve 

 currents, in which galvanometers and exciting circuits are 

 separated by a long tract of nerve, the most complete insulation 

 of the two circuits is no guarantee against the overflow of 

 induced electricity through the interpolar -part of the nerve into 

 the galvanometer circuit. 



There must always be, along with the short-circuiting through 



