STRENGTHS OF FARADIC STIMULI 13 
Sources of Variation. The induction apparatus, as 
used in the physiological laboratory, consists of two cir- 
cuits: the primary, or inducing circuit, which includes 
the primary coil of the inductorium, a source of current, 
and a device for making and breaking the circuit, to- 
gether with the necessary connecting wires; and the 
secondary circuit, including the secondary coil, wires 
leading thence to suitable stimulating electrodes, and 
the tissue to be stimulated. In Fig. 2, p. 6, these cir- 
cuits are illustrated diagrammatically. 
In any given primary circuit variations may arise 
either in the amount of current yielded by whatever 
source of current is used; or in the key, whereby the 
circuit is made and broken. In any given secondary 
circuit variations may arise in the position of the sec- 
ondary coil with respect to the primary, this being, as 
we have seen, the usual method of bringing about varia- 
tions in stimulating strength; in the electrical resistance 
of the tissue which is being stimulated; and in the con- 
tacts between the stimulating electrodes and the tissue 
to which they are applied. Also different inductoria 
usually present structural differences, such as different 
dimensions and different numbers of turns of wire in 
primary and secondary coils, which themselves bring 
about wide differences in the strengths of stimuli gen- 
erated by the different inductoria. The presence or 
absence of an iron core within the primary coil is also 
