964 MESSRS. A. WALLER AND A. DE WATTEYILLE ON THE INFLUENCE 
however be understood that we are dealing with physiological signs alone, and that 
we have not determined the actual physical conditions that underlie them. With 
the method we employ, the actual limits of the provinces of the two effects (anodic 
and kathodic) are a secondary matter, for, admitting the existence of these effects, it 
is sufficient to know that they are coextensive for the two electrical components (test¬ 
ing and polarising) at the common electrode, that the principle of opposite polar and 
peripolar electrotonic states applies to the testing as well as to the polarising current, 
and that as the current density is greater in the polar than in the peripolar province, 
the electrotonic states are proportionately more marked in the former than in the 
latter. 
To secure the coincidence of excitation with polar modification in the case of 
mechanical stimuli, we transmitted the testing blow through the movable electrode 
to a superficial nerve resting On bone. 
The strength of the induction current was altered by altering the distance of the 
secondary from the primary coil of a du Bois apparatus. With the ordinary arrange¬ 
ment (one or two volts for the primary circuit and a resistance of about 1 ohm) the 
make induction shock is hardly effective on the human body, and examination is 
restricted to the effects of the break induction current. In order, therefore, to 
examine also the effects of the make induction current, we adopted the modification 
of Henry,* viz., introduction of resistance into a primary circuit supplied by a battery 
of many cells, in order to obtain make and break induction shocks of about equal 
strength. The interruptions were made by hand.t 
* See du Bois-Reymond, ‘ Gfes. Abhandlungenvol. i., p. 230; Henry, Trans. Phil. Soc., Philadelphia, 
vol. viii., pp. 7, 8 (1840) ; and Phil. Magazine, vol. xviii., p. 488 (1841). 
t This method gives sufficiently regular stimuli, as may he perceived from the figures, care being of 
course taken to make and break contact as uniformly as possible. To this purpose we employed a simple 
spring key in the battery circuit, removing the finger rapidly, so that the break was effected by the 
recovery of the spring. We may remark that whereas by breaking rapidly a more energetic current is 
obtained than by breaking slowly, the contrary obtains in the case of the make, where a more gradual 
contact is more effectual than a sudden one. We have, however, also used a capillary contact with a 
mechanical interruption as a relay without improving upon the regularity of interruption. We prefer the 
manual method because it requires less apparatus, and because the capillary contact is well suited for 
make only. Used for adding to or subtracting from a galvanic current, the latter method would also 
necessitate an undesirable period of short-circuiting, and we used a Helmholtz’ key as will be described. 
