ERRORS TO BE AVOIDED ICQ 
strength due to differences in secondary resistance, 
namely the introduction of a very high additional re- 
sistance into the secondary circuit, thereby making fluc- 
tuations in tissue resistance relatively negligible. That 
this device is perfectly adequate in experiments in which 
a single tissue of varying resistance is under examination 
is of course obvious; there being under such circum- 
stances no variation in cathode surface. But in ex- 
periments in which different tissues are being compared 
the introduction of high additional resistance into the 
secondary circuit is more apt to be misleading than 
otherwise because of the cumulative effect of variations 
in cathode surface. The point can best be illustrated 
by a concrete example: 
Experiment of March 7, 1910. Frog's gastrocnemius muscle stim- 
ulated directly. In the first test the cathode was in contact with the 
surface of the muscle, but did not penetrate it. When the tissue only 
was in the secondary circuit, the total secondary resistance was 17,000 
ohms. A minimal contraction was secured with a value of Z equal to 
6.6. When 70,000 ohms' additional secondary resistance was intro- 
duced, the value of Z was 16.8. By calculation the value of A was 
found'to be 28,000, and that of the specific stimulus /3 to be 4.1-* In the 
second test the cathode was thrust directly through the muscle tissue; 
the secondary resistance was 5400 ohms; the value of Z was 6.1. When 
70,000 ohms' additional resistance was introduced, the value of Z was 
40.5. The calculated value of A was 7000, and of 3-45- In th is case 
the values of Z as determined with the tissue only in the secondary 
circuit represent much more nearly the true relationships between the 
stimuli than do the values as determined with a large additional re- 
sistance in the circuit. In reality the stimulus applied in the first test 
* For the equations used in this calculation see p. 77. 
