ELECTRO-CHEMICAL EQUIVALENT OF SILVER. 
441 
modating to one another the magnitude of the resistance and the strength of the 
current, the electromotive force may be made to balance that of a standard cell, whose 
force is thus determined. The difficulty of the matter relates principally to the 
preparation and definition of the standard cells, and in order to test the constancy of 
the cells it is desirable to extend both the absolute determinations and the comparisons 
of various cells over a considerable range of time. 
Before describing further the arrangements adopted for the absolute measurements, 
it will be convenient to consider the comparisons of E.M.F., which were always made 
by the method of compensation, in order to diminish as far as possible the currents 
actually passed through the cells under examination. The main circuit consisted of 
two Leclanche cells M, and two resistance boxes N, O (joined by a short stout wire) 
of 10,000 ohms each (Plate 17, fig. 1). Of this resistance a variable and adjustable 
proportion was included between the points of derivation, and (by use of the second 
box) the total was in all cases made up to 10,000. Thus, in compensating a single 
Clark cell the resistance from the first box might be 4900, and from the second 
5100. By this means the constancy of the main current is secured. The derived 
branch includes the cell or cells to be tested (P), a mercury reversing key (Q), and a 
galvanometer (T), with which is associated a resistance (S) of 10,000 ohms. The 
galvanometer itself was of the Thomson pattern, and had a resistance of about 200 
ohms. By the substitution of an instrument with a longer wire and of resistance up 
to 10,000, a greater degree of sensitiveness might have been obtained, but with careful 
reading of the galvanometer scale the arrangements were sufficient for the purpose, 
and would indicate the E.M.F. to about t o.oTm j- I 11 the preliminary trials a simple 
contact key with platinum studs was used in the galvanometer branch with the idea 
that shorter contacts would thus suffice. But, probably from thermoelectric disturbance, 
the readings thus obtained were not so consistent as with the mercury reversing key, 
and the smallness of the currents actually allowed to pass rendered the longer contacts 
unobjectionable. From the data already given it will be seen that a current of 10 -6 
amperes was sensible, and no disturbance could be expected from currents 100 times, or 
more, greater than this. In order to test whether the connexions were rightly made, 
the first observation was usually taken with a still higher resistance in the galvanometer 
branch, which could easily be effected by causing the current to pass through the body 
of one of the observers from hand to hand. If by accident too large a current was 
allowed to pass through a cell, no further use was made of that cell until the next 
day. w It must be mentioned that great care was taken, and was necessary, in respect 
of the insulation of the various parts. For instance, no correct results were obtainable 
when the Leclanche’s stood upon the (tiled) floor, if at the same time other parts of 
the combination were touched with the hand. A sheet of paraffined paper interposed 
proved a remedy. In this matter we have had several disagreeable lessons, and we 
* Experiments detailed later (§ 31) show that the precautions observed in this respect were more 
stringent than was really necessary. 
MDCCCLXXXIV. 3 L 
