MAGNETISM AND ELECTRICITY. 186 
until both wires enfeeble equally their proper current. By adjusting the two 
wires until the needle of the galvanometer stands at zero, it will be found 
that the conducting capacity of the two wires is directly as their length, and 
inversely as their cross-section. 
If we complete the same thermo-electric current with two different wires 
in succession, and indicate strength of current, length, cross-section, and 
conducting capacity of one wire by ¢, J, s, c, and that of the other by 
; / 
Se Ge 
Pie st el them = t- Eran thus both currents are equal when 
scl. l=s.c.l. From this we may calculate the length, /’, which a wire 
of cross-section, s’, and conducting capacity, c’, must have, to present the 
same resistance to an electric current as is presented by another whose 
cross-section is s, conducting capacity c, and length 7. Copper is generally 
taken as the standard with which other metals are compared. Deter- 
minations of this character are readily made by means of the apparatus 
shown in fig. 30. Here rr is a thermo-electric element to which are 
soldered two copper wires; these are immersed in the mercury cups 
a and 6, which are connected by a piece of wire, bca, and are united 
besides by a second wire, adb. 
An unalterable thermo-electric current to be used in comparing its action 
on the magnetic needle with the magnetic action of the earth can best 
be obtained by connecting copper and bismuth. This combination should 
consist of a bismuth cylinder (about twenty millimetres in diameter, the 
horizontal part 150 millimetres long, and each vertical extremity fifty 
millimetres), and a copper wire of one millimetre in diameter and twenty 
metres in length. If one place of junction be brought to 32° F., and the 
other to 212° F., this circuit will always give the same current. The copper 
wire is wrapped in twenty windings on a frame, shown in section by 
pl. 22, fig. 31, and from above in fig. 32. The needle which plays on a 
pivot in the centre of the frame is invisible when it is parallel to the 
windings ; for this reason a light plate is fastened to each one on which a 
mark is attached. | 
By connecting several thermo-electric currents in one pile, or compound 
battery, the action will be decidedly strengthened when the first, third, fifth, 
and seventh places of junction are heated, and the intermediate joinings left 
cool, or the reverse. For investigating the laws of such piles, an apparatus, 
as figured in pl. 22, figs. 33 and 34, may be used, consisting of 8, 24, 
or 82 elements of bismuth and copper, as shown in fig. 35. By 
means of glass vessels alternately filled with ice and hot water, the places 
of junction may be kept alternately at 32° and 212° F. <A magnetic needle 
suspended from a silk thread, and held over the middle of a copper 
element, shows by its oscillations the strength of the electric current. <A 
Nobili pile, represented in pl. 19, fig. 37, is well adapted to produce a 
deflection of the needle of an interpolated thermo-electric iultiplier 
(differing from the common one in the smaller number of its windings and 
the greater thickness of its wire). This pile is composed of twenty-five 
to thirty very fine needles of bismuth and antimony, about two inches long, 
357 
