94 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XVII.) 
and 0*4 of an inch internal diameter, open at both ends, bent, and supported on a re- 
tort-stand. In this the liquid was placed, and the portion in the upper part of one limb 
could then easily be heated and retained so, whilst that in the other limb was cold. 
In the experiments I will call the left-hand side A, and the right-hand side B, taking 
care to make no change of these designations. C and D are the wires of metal (1881 .) 
to be compared ; they were formed into a circuit by means of the galvanometer and, 
often also, a Seebeck’s thermo-element of antimony and bismuth ; both these, of 
course, caused no disturbing effect so long as the temperature of their various junc- 
tions was alike. The wires were carefully prepared (1881.), and when two of the 
same metal were used, they consisted of the successive portions of the same piece of 
wire. 
1916. The precautions which are necessary for the elimination of a correct result 
are rather numerous, but simple in their nature. 
1917- Effect of first immersion. — It is hardly possible to have the two wires of the 
same metal, even platinum, so exactly alike that they shall not produce a current in 
consequence of their difference ; hence it is necessary to alternate the wires and 
repeat the experiment several times, until an undoubted result independent of such 
disturbing influences is obtained. 
1918. Effect of the investing fluid or substance . — The fluid produced by the action 
of the liquid upon the metal exerts, as is well known, a most important influence on 
the production of a current. Thus when two wires of cadmium were used with the 
apparatus, fig. 7, (1915.) containing dilute sulphuric acid, hot on one side and cold 
on the other, the hot cadmium was at first positive, producing a deflection of about 
10°; but in a short time this effect disappeared, and a current in the reverse direction 
equal to 10° or more would appear, the hot cadmium being now negative. This I 
refer to the quicker exhaustion of the chemical forces of the film of acid on the heated 
metallic surface (1003. 1036. 1037-), and the consequent final superiority of the colder 
side at which the action was thus necessarily more powerful (1953, &c. 1966. 2015. 
2031, &c.). Marianini has described many cases of the effects of investing solutions, 
showing that if two pieces of the same metal (iron, tin, lead, zinc, &c.) be used, the 
one first immersed is negative to the other, and has given his views of the cause*. 
The precaution against this effect was not to put the metals into the acid until the 
proper temperature had been given to both parts of it, and then to observe the first 
effect produced, accounting that as the true indication, but repeating the experiment 
until the result was certain. 
1919. Effect of motion . — This investing fluid (1918.) made it necessary to guard 
against the effect of successive rest and motion of the metal in the fluid. As an illus- 
tration, if two tin wires (1881.) be put into dilute nitric acid, there will probably be a 
little motion at the galvanometer, and then the needle will settle at 0°. If either wire 
be then moved, the other remaining quiet, that in motion will become positive. Again, 
* Annales de Chimie, 1830, xlv. p. 40. 
