THE MOLECULAR THEORY OF EXCITATION 607 



by an external agent, as modifying the intensity of the 

 transmitted effect. In order to study the phenomenon of 

 conduction and its modification, as will be remembered, a 

 delicate form of Conductivity Balance, fully described in 

 Chapter XXXIII. was used. Excitation was here caused by 

 S at a middle point, the transmitted excitatory effects at E' 

 and E being made to balance, This condition of balance 

 was obtained when one arm, say the left E', was kept at a 

 fixed distance from S, and the other, or right, was moved 

 towards S, or away from it, as required. When E was too far 

 from S the excitatory effect would be smaller than at E', and 

 this under balance would be indicated by a response, say 

 downwards. When, again, E was too near to S, there would 

 be an over-balance, the resultant response being upwards. 

 Between these could be found a point of exact balance 

 where the record was horizontal (cf. figs. 289, 290). 



A high degree of delicacy in the study of similar 

 phenomena in the case of iron wires may be obtained by 



FIG. 372. Magnetic Conductivity Balance 



s, magnetising coil, by which north-polar or K-impulses are sent out in 

 two directions as shown by arrows. E E', receiving coils, adjusted 

 at balance. M, permanent magnet, by which either A- or K-tonus 

 is induced at the responding ends of the iron rod. T, tonic coil, 

 by which A- or K-tonic molecular dispositions may be induced in 

 one arm of the balance. 



the employment of the Magnetic Conductivity Balance 

 (fig. 372), which I shall now describe. The magnetic 

 stimulator, S, consists of a pair of similar coils wound in 

 opposite directions. When a magnetising current is suddenly 



