16 
PROFESSOR B. SANDERSON ON THE ELECTROMOTIVE 
C. For the preliminary investigation of the time relations of the electromotive changes 
which follow excitation the most valuable instrument is the capillary electrometer. 
Two important improvements have been recently made in the construction of this 
instrument by my friend Professor Loven/' of Stockholm. The first of these consists in 
constructing the tube which contains the dilute sulphuric acid, of extremely thin glass 
so that the capillary tube which is in contact with its internal surface can be observed 
with a magnifying power of at least 300 or 400 diameters (No. 10 Hartnack). The 
other consists in cementing the sulphuric acid tube to the mercury tube, so that no 
evaporation takes place. By the first, the instrument is rendered much more sensitive, 
by the second, variations in the strength of the sulphuric acid which arise from 
evaporation are avoided. When used for the purpose of observing a rapid succession 
of changes in the electrical relations of two led off surfaces, its superiority to the 
galvanometer is very striking. The movements of the mercurial column not only 
correspond closely in time to the actual changes which they represent, but express 
with very great accuracy the differences of potential which actually exist in each 
successive phase of a variation. In the beginning of an investigation this is invaluable, 
for the summary information of the duration, character, and progress of the electrical 
disturbance produced by excitation, which the electrometer affords, renders it possible 
to avoid loss of time in the subsequent application of more accurate modes of measure¬ 
ment. For the rougher kinds of time measurement, the use of the electrometer may 
be combined with that of the electro-magnetic signal, in the manner explained in our 
former paper. By this means the occurrence of an event can be determined within a 
tenth of a second by a single observation, and, of course, much more accurately by 
repetition. As a means of measuring electromotive force, the electrometer may be 
used with great advantage when the electrical states to be investigated are transitory, 
and at the same time so irregular that the rheotome could not be applied. For this 
purpose it is much better to measure the electromotive value of each excursion with 
the aid of the compensator than to deduce it, as has been done by Fleischl,! from the 
“compensation pressure/’ With this exception the measurements of electromotive 
force can be made much more accurately with the galvanometer. If it were possible 
to photograph the motions of the mercurial column of the electrometer it would be an 
addition to its usefulness. 
Use of the Pendulum Myograph as a Rheotome .—The pendulum myograph (fig. 7) 
is essentially a seconds pendulum, the weight of which carries a glass plate so fixed 
pensator, G the galvanometer, E the capillary electrometer. The wires marked with arrows lead to a 
battery. K, K 1 , &c., are keys. W is a switch by which the wires from the electrodes may be connected 
either with the galvanometer (as shown in the diagram), or with the electrometer (as when the bridge is 
turned to the right), w is the reverser connected with the compensator shown in fig. 5. 
* Loven, “ Om kapillarelektrometern och kviksilfertelefonen,” Nordiskt medic. Arkiv. vol. xi., No. 14. 
f v. FleiSCHL, “ Ueber die Construction und Verwendung des Capillar-Electrometers, fur electrophy- 
siologische Zwecke.” Archiv. f. Anat. u. Physiol., 1879, p. 269. 
