CHAPTER VI 
EFFECTS PRODUCED BY AN IRON CORE IN THE 
PRIMARY COIL 
INASMUCH as the almost universal practice in physio- 
logical work is to use inductoria with iron cores, a brief 
discussion of the effects of such cores on stimulation 
strengths seems desirable at this point. Thus the 
method becomes at once applicable to inductoria with 
iron cores as well as to those not provided with them. 
The principal effect of the iron core is that which has 
led to its use, namely a great increase in the number of 
lines of force surrounding the primary coil, with a cor- 
responding increase in the intensity of the stimuli gen- 
erated. 
Another effect is that noted in a previous paragraph 
(p. 36), of altering the effectiveness of the stimuli gen- 
erated when the secondary coil is directly over the pri- 
mary, so that in these positions M I is not constant for a 
constant stimulus. The method of correcting the cali- 
bration for this effect of the iron core is given in Chap. 
VIII, p. 58. 
The iron core has also an effect upon stimulation 
strength due to its magnetization by the primary current, 
an effect which appears, however, only when primary 
currents of considerable intensity are used. Allowance 
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