A. M. Mayer on measuring Electrical Conductivities. 315 
If a galvanometer formed of 6 or 8 turns of ‘1 in. wire were 
used in connection with a powerful magnetic battery and larger 
spirals of thicker wire, while the galvanometer is placed at a 
greater distance, I have no doubt that a variation of ;;'55 part 
can thus be detected and measure 
5. Examples of the determinations of electrical conductivities by 
The object of these determinations was not to furnish science 
with new and accurate data,—for that would have required a 
careful personal supervision ‘of ee operations of preparing chem- 
ically pure metals,—but it was to give examples setting forth 
the practice of the method. 
had prepared “hard-drawn” wires, of No. 18 B. W. G. 
(=-049 in. diam.), of seg ea core iron, and German silver. 
These wires were found to have the anes diameter. They were 
all covered with a double fem gato of s 
he spirals were balanced, by oe introduction of an 
increased r resistance in the back- -spiral, so that no deflection took 
place on sliding off the spirals. A length of 120 in. of the sil- 
ver wire having been placed in the circuit of one spiral, it was 
found that 127 in. of copper wire were required in the other 
circuit, in order to equal it in resistance. oe. the copper 
wire as the standard of comparison, at 100, we hav: 
127: 120 :: 100:: 94°48. 
Matthiessen (Phil. Trans., 1858, 1862) makes the ratio - sre 
conductivity of silver to copper, both hard-drawn, as 100: 
or about equality ; but in my determination the silver is 5 5. ‘de 
cent below the copper. I therefore suspected impurities in the 
silver, and an examination of the wire Eindly made by rs col- 
league, Dr. Wetherill, showed that it contained about ‘01 per 
cent of gold anda trace of iron. This accounts for the low 
number found, and affords a good illustration of Pouillet’s re- 
mark, that the purity of a metal is most readily determined by 
a measure of its electrical conductivity. The electrical test of 
parity; however, exceeds in — the chemical examination ; 
a very minute percentag causes a great increase of 
resiaianes and if we could see sure at the wires we com 
were in the same physical condition as to annealing 7 - 
ness, we could atneie d ee. this method as a means of deter- 
mining the wn metal which formed the alloy. 
Pouillet anes: (itaité de P de Physic 2 1856, vol. i, p 606) that 
silver whose conductivity is 100 when pure, is only 51 when it 
contains 037 of alloy, and is 47, "2 and 389 when it contains 
respectively *100, “143, and -258 of alloy. Pure gold gave 39, 
but 049 of alloy reduced its conductivity to 13; and Jenkin 
