THE INFLUENCE OF SECONDARY RESISTANCE 77 
application of the fact has hitherto been reserved for 
galvanic stimulation. The expression for shows that 
current density is a factor to be taken into account in 
measuring faradic stimuli as well. 
ZA 
The factor A in the expression /3 = is the pro- 
R -f~ A. 
vision by which allowance is made for the influence of 
current density. The stimulating effectiveness of dense 
currents is greater than of diffuse ones. In order that 
the expression for /3 agree with this fact the value of A 
must increase as the density of the stimulating current 
increases. Experimental evidence showing that A is 
actually larger for dense currents than for diffuse cur- 
rents is contained in an experiment cited on p. 109, 
Chap. XII. 
I do not know of any reliable method of determining 
the value of the constant, A, other than that used in 
this work, namely to establish experimentally two or 
more values of Z for different secondary resistances, 
and from these values compute the value of A. This 
can be done by means of the equation 
Z R R f Z R >R ( 
A = j ^ ' (3) 
^w R 
in which Z# and Z K are the stimuli required, with re- 
sistances R and R f respectively, to produce the minimal 
contractions used as the index. 
