EFFECTS OF IRON CORE IN THE PRIMARY COIL 47 
tion shocks, since thereby is avoided that heavy sparking 
at the contacts which always accompanies the break of 
a current of high intensity, and which affects the 
intensity of the stimulus in a manner that cannot be 
foretold. 
When the secondary coil of an inductorium is moved 
from the zero position until nearly clear of the primary 
coil, it enters a " critical region" where small changes in 
position are accompanied by great changes in the in- 
tensity of the stimuli given by the instrument. The 
impression seems to prevail among physiologists that 
inductoria having iron cores show so much greater vari- 
ations of intensity in this " critical region" than do those 
without iron cores as to make the omission of the iron 
core a distinct advantage in many experiments. As a 
matter of fact, however, Kronecker inductoria, such as 
are used in most physiological laboratories, show for 
given changes in secondary position in the " critical 
region" greater variations in stimulation intensity with 
cores removed than with cores present. This is appar- 
ent when the Kronecker graduations of such coils are 
compared with the calibrations made for them by the 
method of the present work (see p. 55). In the prep- 
aration of the Kronecker graduations the iron cores 
were withdrawn from the instruments. For the cali- 
brations made in connection with this work the iron 
cores were in place. 
