CHAPTER VII 
COMPARISON OF ONE COIL WITH ANOTHER THE 
VALUE OF L 
WE have seen (p. 41) that in any given inductorium, 
after allowing for certain exceptions due to the iron core, 
if one is present, the strengths of stimuli produced by a 
given primary current with the secondary coil at various 
positions are directly proportional to the mutual induc- 
tions for those positions. When, however, the attempt 
is made to compare the stimuli generated by one in- 
ductorium with those produced by another, it is at 
once apparent that the relation between stimulating 
value and mutual induction holds only for stimuli pro- 
duced by the same instrument. This, indeed, was rec- 
ognized by Helmholtz, who pointed out the necessity of 
including in the expression for stimulating value the 
factor L, whereby to take account of the influence of 
inductorium construction. This factor, according to 
Helmholtz, is dependent on the inductance of the sec- 
ondary coil, and is to be derived, therefore, from the 
expression for inductance. The common formula for 
AW 2 
the inductance of a coil is L = > in which L is the 
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