Equipotential Lines of an Electric Current. 429 
On the other hand, the decrease in the observed value of the 
R. P. as M rises from 5835 to 8651, though slight, seems to me 
strong evidence of an actual falling off in the value of the 
R. P. of iron at magnetic intensities. 
In any case it appears, and this was one of the important 
questions to be answered by the experiments, that the R. P. of 
iron under magnetic forces of varying intensity is more nearly 
constant than the R. P. of nickel, for according to previous 
experiments* the R. P. of nickel in a magnetic field of in- 
tensity 8700 is many per cent., perhaps 20, less than in a 
magnetic field of intensity 2000. 
The one trial made at low temperature confirmed the in- 
ference drawn from a similar trial made the year beforef, 
that a fall of 1° C diminishes the R. P. in soft iron about 2 
per cent. It is with these experiments as a basis that I have 
ventured to make the slight changes necessary to deduce from 
the observed results the values of the R. P. at 28°. 
A test was made with this strip of soft iron for a permanent 
change in the direction of the equipotential lines. A similar 
experiment with tempered steel the year before had been suc- 
cessful, and the description of the method I will quote froma 
previous article (Philosophical Magazine for May 1883, p.345). 
“This plate, with the usual electrical connections, and with a 
_ current flowing through it, was placed in the usual position 
between the poles of the electromagnet, the magnet current 
was turned on, then off, and the plate removed from between 
the poles in order to avoid the action of the very considerable 
residual magnetism of the electromagnet. A reading of the 
Thomson galvanometer in the transverse circuit was now made, 
then the plate was replaced between the poles and the current 
turned on again, but in the opposite direction. The magnet 
current being again interrupted, the plate was again removed 
from the field and another reading of the Thomson galva- 
nometer was made.” Making this experiment with the soft 
iron and with a battery of 50 cells, the result was a negative 
one. If any permanent effect was produced, it was probably 
a small part of one per cent. of the temporary effect produced 
while the plate was subjected to the magnet’s action. 
Steel. 
The steel used in this series of experiments was obtained 
from Monigomery and Co., New York, and was designated 
by the letters “F.C. R.,” which stand for “ French Cold 
* Phil. Mag. Sept. 1881. 
+ Phil, Mag. May 1883. 
Phil. Mag. 8. 5. Vol. 19. No. 121. June 1885, 2G 
