THE INFLUENCE OF SECONDARY RESISTANCE 73 
polarization, and a telephone in place of the galvanom- 
eter. Figure 15 is a diagram of the apparatus required. 
The average of three readings was always taken. This 
procedure, in the hands of one experienced in its use, 
gives results accurate within 4 or 5 per cent, a degree 
of accuracy sufficient for the purposes of this inquiry. 
FIG. 15. Diagram of apparatus for measuring tissue resistance. A, 
Wheatstone bridge; B, telephone; C, small induction coil; D, battery for 
same; E, key for same; T, wires leading to tissue; R, resistance box con- 
nected with switch, S, in such fashion as to be available for use as 
known resistance of Wheatstone bridge, or as part of primary circuit, P. 
. Break shocks were used for determining the threshold 
of contraction. The expression for the value of the 
stimulus is Z, determined from the formula, Z = 7. 
it 
The Effect upon the Stimulus of Varying the Sec- 
ondary Resistance. The effect upon the value of Z of 
varying the secondary resistance is shown in two repre- 
sentative experiments cited in Table VI. As appears 
