MODIFICATION OF THE DISTRIBUTION OF POTENTIAL BY THE 
'EXTERNAL ARC ' 
Changes in the Distribution of Potential Produced by the 
Conditions of an Experiment 
A necessary step in the process of examination of the nerve is the application to 
it of two electrodes and a connecting wire path. The whole arrangement with the 
included galvanometer may conveniently be termed an observation circuit, or, simpler 
still, an ' external arc' It is a well-known fact that the distribution of potential on 
the surface of the nerve, as on any other conductor, is unaffected by the presence of 
such an external arc when the current found traversing it has been accurately com- 
pensated. It is also a well-established, but by no means so generally known, fact 
that the presence of such an external arc profoundly modifies the distribution of 
potential upon the surface and in the interior of the intervening piece of nerve, when 
the current traversing the arc is left uncompensated. 
The first part of this statement, namely that the external arc with compensated 
currrent is not a disturbing factor, is of obvious importance to the actuality of the 
results obtained by this method of examination, and it was shown by Helmholtz 1 
to be mathematically true. The value of the error due to an absence of compensation 
was also calculated by him, and that, too, with a special reference to the case of animal 
tissues and the centres of electromotive activity, assumed by Du Bois Reymond to 
be imbedded in them ; the calculation showed that the distribution of potential was 
modified in a very precise manner. The effect of placing an external arc upon such 
a conductor, and the consequent derivation of a current through the arc, was shown 
so to alter the distribution of potential in the conductor as if the conductor formed 
a portion of the circuit through which the current through the arc was flowing. Thus 
in the case of a nerve, the current flowing in the galvanometer circuit from longitudinal 
surface to cross section exactly modifies the pre-existing distribution of potential in 
the nerve, as if it also flowed in the nerve from cross section to longitudinal surface. 
The truth of the calculations was also demonstrated in experiments performed 
upon conductors in which sources of EMF were placed; but the actual demon- 
stration of the quantitative influence of this condition in experiments upon nerve has 
never been undertaken. The presence of the condition has, indeed, been noticed 
in physiological experiments, as, for example by Von Fleischl, 2 in observations 
I. For a detailed account of Helmholtz's postulates see Adolf Fick, Die Medivunhche Physik, 1858, p. 354 ; 
Morgan, Elcctrophysiology, etc., p. 265. New York, 1868. 
2. E. v. Fleischl, Electrotonus, etc., Sitzungber d. Wien, Acad., LXXVIII, Abth. 3. 
