And strain on the action of physical forces. 
45 
Preliminary trials and final method of experimenting. 
In the first attempts which were made the galvanometer was put into circuit by 
means of the usual contact-piece of the sliding-block H, immediately after closing the 
battery ; but it was soon found that the-very act of pressing down the contact-piece 
generated small thermo-electric currents' 5 ' which, lasting some minutes, frustrated all 
attempts at making such accurate measurements as it was hoped would ultimately be 
obtained. H was therefore clamped by means of a suitable spring-and-catch, with which 
it was provided, to any desired part of the platinum-iridium wire, and a double key was 
employed by means of which first the battery and then immediately afterwards the 
galvanometer were put into the “ bridge.” Here again, however, exactly the same 
difficulty was encountered, and by no device of covering the hand and key with cloth or 
silk could this source of error be entirely avoided. The double key was therefore 
discarded, and the galvanometer being always kept in the “ bridge,” a single key was 
used for closing the battery circuit for the brief space of time necessary for observing 
whether this act caused any difference of potential at the terminals of the galvanometer. 
Of course the zero-point of the galvanometer needles was continually being altered 
by the thermo-electric currents produced by the frequent shifting and clamping of H, 
but this circumstance did not affect the results, and though in the case of iron a slight 
trouble was experienced sometimes from the “ kick ” of the needles due to circular 
magnetization, this difficulty was after a few trials surmounted, and from this point the 
measurements proceeded very satisfactorily. 
In most instances a and /3 were made of about 100 ohms’ resistance, and though such 
large resistances were, it is true, out of proportion to those of the other branches of the 
bridge, yet, as has been already observed, the arrangement proved of amply sufficient 
delicacy, and moreover rendered it impossible that any slight variations of the resistances 
of X and Y, which were each '042 ohm, and of the wire N N, arising from changes of 
temperature, should cause any appreciable error ; indeed, one great advantage of this 
method is that the galvanometer and the resistance-coils may be a hundred yards or 
more from the rest of the “ bridge ” without any chance of fluctuations of temperature 
materially influencing the result, even when the most minute variations of electrical 
conductivity are to be measured. 
The deflections of the galvanometer were read in the usual manner by means of the 
image of a fine wire fixed vertically across one end of a small blackened tube, into the 
other end of which was fitted a lens for focussing the image of the wire on to the mirror 
of the galvanometer; and the end of the tube at which the wire was situated was 
illuminated by a paraffin lamp, placed so that the edge of the flame was in front of the 
wire, the reflected circle of light with the fine dark line across the centre being very 
* This fact has, I find, been also noticed by Gi-azebrook, Phil. Mag., April, 1881, No. G8. 
