THE CHARACTERISTICS OF INDUCED CURRENTS 3 
vised, but for physiological purposes only one has come 
into common use; this form, designed by E. du Bois- 
Reymond * in 1848, is illustrated in Figs, i and 2. 
Such modifications of this design as have arisen since 
its introduction have to do only with details, and not at 
all with the underlying principle of the apparatus. 
Structure of the Inductorium. The induction coil, as 
adapted by du Bois-Reymond to physiological use, con- 
FIG. i. The induction coil as used for physiological purposes 
(du Bois-Reymond pattern): A, the primary coil; B, the secondary 
coil; P r , binding posts to which are attached the wires from the battery 
they connect with the ends of coil A; P", binding posts connecting 
with ends of coil B, through which the induction current is led off; S, 
the slide, with scale, in which coil B is moved to alter its distance from A. 
sists, in essence, of two coils of carefully insulated copper 
wire. One of these, the primary coil, is made up of 
two or three layers of rather coarse wire wound upon a 
hollow core of nonconducting material. Usually the 
outside diameter of this coil is about 2.5 to 4 cm., and 
its length between 8 and 14 cm. The number of turns 
* du Bois-Reymond: Unters. iiber tierische Electrizitat, 1848, Bd. 1, 
S. 4471 also, Bd. II, i, S. 393. 
