Equipotential Lines of an Electric Current. 427 
. Per cent. Per cent. 
Specimen. | org opper. Oe pivies Temp. R. P. 
A. No. 2. 99-9 yas Nf 100(—) 
B. i 81:08 18°51 21° 76°1(—) 
ited : i 4° | 787(—) 
G. % sex 99°54 die 158°8(++) 
A No. 2 and B No. 2 were compared at about 25° C., 
A No. 2 and G No. 2 at about 19° C. 
The values of R. P. given in the first table are in the 
following proportion :— 
R. P. 
ee ele 1218) Sot LOO (=) 
Ba ee ek tebe) 
Cree er CO) 
This agreement was regarded as highly satisfactory, all 
things considered ; and the apparent exception which, as stated 
above, had been noted in the case of B, appeared to be con- 
firmed. The result of the chemical analysis, however, showed 
that this alloy, which, according to the label found upon it, 
contained nearly 92 per cent. of copper, really had the com- 
position given in the tables above. Alloy B, then, does con- 
form to the rule that the alloys of copper and zinc have 
rotating powers nearer to that of copper than the composition 
of the alloys would have led one to expect. 
Influence of Temperature. 
It will be seen that in the case of copper, zinc, and all the 
alloys except C,a fall of temperature appears to cause a slight 
increase in the numerical value of the rotating power. It is 
possible that upon further trial the apparent exception fur- 
nished by C would disappear. It does not, on the other hand, 
seem probable that the agreement in a particular direction of 
five cases out of six is entirely accidental. In the case of D 
the apparent change caused by fall of temperature is very 
considerable ; but the observations made with this alloy were 
particularly unsatisfactory, the needle of the Thomson galva- 
nometer being quite unsteady, while the total effect to be 
measured was small. 
_ The experiments upon iron and steel were in the main 
repetitions of those already published, but made with more 
care. 
