264 



COMPARATIVE ELECTRO-PHYSIOLOGY 



prepared as to be anisotropic, one side having a greater 

 potentiality of galvanometric negativity under excitation 

 than the other. In that case, further, it was clear that the 

 strongest resultant current would be obtained, if one surface 

 of the structure became galvanometrically positive, and the 

 other negative, on excitation. I have already stated, in 

 Chapter I., that different inorganic substances give electrical 

 responses of opposite signs. Thus the response of lead is 

 positive, while that of brominated lead is negative. If, then, 



we take a lead wire A B, 

 clamped in the middle 

 at C, as represented 

 in the upper diagram 

 (fig. 165), and stimulate 

 the right-hand end B, 

 say by mechanical 

 vibration, a responsive 

 current will be induced, 

 which will flow to- 

 wards the stimulated, 

 B thus becoming gal- 

 vanometrically posi- 

 tive. The same will 

 be the case with A, 

 on stimulation. When 

 both A and B are simul- 

 taneously stimulated, it is evident that the two responsive 

 currents, being antagonistic, will cancel each other. But 

 if the right-hand half B' of the wire consist of brominated 

 lead (lower diagram, fig. 165), while the left-hand half A' is of 

 lead like that used in the first case, then stimulation of B' will 

 cause a responsive current to flow away from the right-hand 

 excited end, B f thus becoming galvanometrically negative ; 

 A', on the other hand, will give rise to a positive response 

 towards the excited. Simultaneous excitations of A' and B' 

 will not then be antagonistic, but additive in their effects. 

 The resultant response will thus be from the negative B' to 



FIG. 165. Responsive Currents in Lead Wire 



Upper figure Excited wire, galvanometrically 

 positive. Simultaneous excitation of both 

 ends balance each other ; resultant response 

 zero. 



Lower figure Left portion, lead wire, right 

 portion, brominated lead wire, shown as 

 shaded. Response of first positive, of second 

 negative. Simultaneous excitation of the 

 two induces resultant response which is 

 additive, from the right to the left. 



