252 COMPARATIVE ELECTRO-PHYSIOLOGY 



at which the E.M.F. attained its maximum. The general 

 arrangement here is similar to that described in Chapter IV. 

 (cf. fig. 37). C is the compensator by which any existing 

 electro-motive difference is compensated at the beginning of 

 each experiment. The striking- rod A breaks the current 

 in the electro-magnet E, by which the rotating reverser R is 

 actuated, which brings about equal and opposite shocks to 

 the leaf. The galvanometric after-effect, at any short 



FIG. 159. Experimental Arrangement for Rheotomic Observations 



A, B, striking-rods attached to revolving rheotomic disc; K,, key for 

 electro-magnetic release of rotating reverser R ; K 2 , key for unshunting 

 the galvanometer when pressed by rod, B, for a definite period ; K 3 , 

 key for preliminary adjustment ; E, electro-magnet with its armature 

 by which rotating reverser, R, is set against antagonistic spring, s ; 

 K 4 , Pohl's commutator; c, compensator; P, primary, and I, the 

 secondary, of the exciting induction-coil ; N,, N. 2 , non-polarisable elec- 

 trodes, making electrical contacts with posterior and anterior surfaces 

 of leaf. 



interval after excitation, is obtained by the un-shunting of 

 the galvanometer, caused by the striker B impinging against 

 the key K 2 (fig. 159). We have seen that, owing to the 

 presence of various complicating factors, as well as to the 

 occurrence of negative polarisation, successive responses to 

 homodromous and heterodromous shocks are unequal. By 

 the employment of equi-alternating induction-currents, how- 



