VOLTAIC EXCITATION AFFECTED BY TEMPERATURE — PRECAUTIONS. 
97 
flection of a galvanometer needle in the circuit, from 1 2° to 30° or 45° ; whilst heat 
applied to the positive electrode caused no change*. I have not been able to obtain 
this nullity of effect at the positive electrode when a voltaic battery was used (1639.); 
but I have no doubt the present phenomena will prove to be virtually the same as 
those which that philosopher has described. 
1927. The effect interferes frequently in the ensuing experiments when two metals, 
hot and cold, are compared with each other ; and the more so as the negative metal 
approximates in inactivity of character to platinum or rhodium. Thus in the compa- 
rison of cold copper, with hot silver, gold, or platinum, in dilute nitric acid, this effect 
tends to make the copper appear more positive than it otherwise would do. 
1928. Place of the wire terminations. — It is requisite that the end of the wire on the 
hot side should be in the heated fluid. Two copper wires were put into diluted solu- 
tion of sulphuret of potassium, fig. 8. ; that portion of the liquid extending from C to 
D was heated, but the part between D and E remained cold. Whilst both ends of the 
wires were in the cold fluid, as in the figure, there were irregular movements of the 
galvanometer, small in degree, leaving the B wire positive. Moving the wires about, 
but retaining them as in the figure, made no difference ; but on raising the wire in A, 
so that its termination should be in the hot fluid between C and D, then it became 
positive and continued so. On lowering the end into the cold part, the former state 
recurred; on raising it into the hot part, the wire again became positive. The same 
is the case with two silver wires in dilute nitric acid ; and though it appears very 
curious that the current should increase in strength as the extent of bad conductor 
increases, yet such is often the case under these circumstances. There can be no 
reason to doubt that the part of the wire which is in the hot fluid at the A side, is at 
all times equally positive or nearly so ; but at one time the whole of the current it pro- 
duces is passing through the entire circuit by the wire in B, and at another, a part, 
or the whole, of it is circulating to the cold end of its own wire, only by the fluid in 
tube A. 
1929. Cleaning the wires. — That this should be carefully done has been already 
mentioned (1881.) ; but it is especially necessary to attend to the very extremities of 
the wires, for if these circular spaces, which occur in the most effective part of the 
circle, be left covered with the body produced on them in a preceding trial, an experi- 
mental result will often be very much deranged, or even entirely falsified. 
1930. Thus the best mode of experimenting (1915.) is to heat the liquid in the limb 
A or B, fig. 8., first; and, having the wires well cleaned and connected, to plunge 
both in at once, and, retaining the end of the heated wire in the hot part of the fluid, 
to keep both wires in motion, and observe, especially, the first effects : then to take 
out the wires, reclean them, change them side for side and repeat the experiment, 
doing this so often as to obtain from the several results a decided and satisfactory 
conclusion. 
* Bibliotheque Universelle, 1837, vii. 388. 
MDCCCXL. 
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