OF THE GALVANIC CURRENT ON THE MOTOR NERVES OF MAN. 
965 
The strength of the polarising current was varied by altering the number of cells 
(freshly charged Leclanche’s with conglomerate plates), and to avoid fallacies arising 
from changes of resistance occurring during experiment, controlled by a galvanometer 
constructed to measure from 1 to 50 thousandths of a B.A. unit (milliwebers). In 
order further to eliminate the error caused by changes of resistance (vide infra) we 
introduced an additional resistance into circuit, maintaining the current at its original 
strength by employing greater electromotive force (e.g., by 4000 ohms + galvano¬ 
meter resistance about 750 ohms, i.e., three times that of the average resistance 
of the body in the conditions of our experiments, the possible effects of changing 
resistance are divided by four). By this device we. ascertained that our results 
were practically independent of alteration of current strength by alteration of 
resistance. It occasioned, however, the observation that addition of resistance in 
the secondary circuit diminishes the effect of the anode of the induction current 
(make or break) far more markedly than that of the kathode, a point which is 
illustrated in Tracing 32. 
We observed a fact of the same kind with the Henry modification of the current, 
viz.: that on gradually increasing the strength of current by bringing the secondary 
nearer to the primary coil, a break positive effect, which appeared with lower strength, 
was gradually overtaken by the corresponding make negative effect, which under 
certain circumstances might even surpass the break positive effect with higher 
strength (Tracing 83). We found the former phenomenon to be independent of the 
nature of the resistance apparatus used, and of its position in the circuit. We did 
not find anything analogous for the anode and kathode of the continuous current, 
the diminution of the contraction at the anode being parallel to that at the kathode 
when the resistance in circuit is augmented ; on the other hand, we noticed that when 
wire coils are used, with the same current strength, the make effect at both poles is 
greater without than with additional resistance in circuit, the break effect remaining 
unaffected. This diminution of the make effect is doubtless due to the generation 
of an extra current in the resistance coil. 
To record the muscular contractions which give the measure of nerve-excitability, 
we employed a small thick indiarubber bag, strapped to the part chosen for exploration, 
and connected by a flexible tube with a Marey’s tympanum. The excursions of the 
lever were recorded on a smoked cylinder travelling at a convenient speed (about 
2 centims. per minute). We preferred this mode of estimating excitability by 
strength of contraction, to that of estimating it by minimum stimulus, and we only 
determined the minimum stimulus for the sake of comparison. Whether during 
polarisation, increase seen on the figure was due to increased contraction of a given 
amount of muscle, or to the added contraction of previously unexcited muscle, or 
to both factors, does not affect the evidence for our present purpose. We did not, 
strictly speaking, measure the nerve excitability, but only determined whether it 
