76 INDUCTION SHOCKS 
for the experiment of Table VI from which that curve is 
derived is 3. Since this represents the value of Z, whose 
effect at zero resistance would equal that of the various 
other values of Z at their respective resistances, it 
affords a measure of the irritability of the physiological 
cathode where the stimulus actually arose, on the as- 
sumption that the resistance of the cathode is negligibly 
small. We have, therefore, in an expression for the 
value of any stimulus as it affects the seat of actual 
stimulation, namely, the physiological cathode, irre- 
spective of the resistance of the secondary circuit. 
By a slight transposition of equation (i) the equation 
for /3 becomes: 
ZA . . 
'-R + A' (2) 
and if the value of Z for any secondary resistance is 
known, the actual or " specific" stimulus can be calcu- 
lated from equation (2), provided only the value of the 
other constant, A, is known. For measuring stimuli 
with reference to the resistance through which they are 
applied there must be added to the determinations 
previously required, therefore, not only the secondary 
resistance, but a constant A. 
Current Density an Important Factor. That the stim- 
ulating effectiveness of electric currents varies with their 
density has long been recognized,* although practical 
* Biedermann: Loc. cit., i, p. 185. 
