58 INDUCTION SHOCKS 
origin of the muscle was in most cases made the cathode. 
With the minimal contraction of the muscle as the index, 
the primary current necessary to arouse it, measured in 
amperes, is determined with the secondary coil in various 
positions. To allow for variations in irritability of the 
tissue the experiment should be repeated a number of 
times. If the calibration is carefully made in the be- 
ginning it will be found that in each individual experi- 
ment the product - X /, primary current times cali- 
Lt 
bration number, is virtually constant, showing that 
the calibration is correct. 
Should the inductorium being calibrated have an iron 
core, there still remains the establishment of calibra- 
tion numbers for the region where the secondary coil 
overlaps the primary. These, however, can easily be 
determined by extending the experiments, just de- 
scribed for proving the calibration, to cover this part of 
the field. The value of - X / is established in any 
.L/ 
M 
given experiment from the part of the field where is 
Li 
known, that is, where the calibration has already been 
worked out. Since this is constant so long as the stim- 
ulus is unchanged a determination of the primary current, 
M 
7, for this stimulus, in the region where is unknown, 
