Magnetic Permeability of various Alloys of Iron. ral 
different places by means of a micrometer-screw reading to one 0:01 millim., and 
the mean of the readings was taken as the true diameter: the diameters of the 
standards were similarly determined once for all.* The conductivity of the test 
rod was calculated in terms of pure copper, Matthiessen’s standard of pure 
copper being taken as 100; our own specimen was 1:01 per cent. higher con- 
ductivity than Matthiessen’st; allowance for this was made in the following 
calculation for the conductivity of the rods under test :— 
1 = length of standard. 
!’ = length of rod under test. 
d = deflection due to fall of potential in standard. 
d’ = deflection due to fall of potential in test rod. 
a = cross-sectional area of standard. 
a = cross-sectional area of test rod. 
conductivity of standard. 
= conductivity of test rod. 
F dla 
= Fra 
~ 
YX ® 
Il 
The following comparison of the iron standard with the copper standard will 
serve as an example of the series of readings taken with all the rods :— 
Stanparp Iron (Temperature 15° C.). 
Mark. 
aloe | a ie 
) 
| 
801. 140°5 | 54°8 | 0:0121 | 0:1874 | 100 
from which the conductivity ¢’ was deduced thus :— 
p _ Ud Oe s¢ OU 
C= maa ld Ome oe 
or conductivity of standard iron = 16°35, pure copper being taken as 100. 
* The cross-section of the rods was slightly elliptical, and, moreover, not quite uniform throughout 
their length ; later on in the investigation, the mean cross-sectional area of each rod was more accurately 
and expeditiously determined by measuring the length of the rod carefully, and then obtaining its volume 
by displacement of water in a long glass tube, 1 cm. diameter, graduated to tenths of ac.c. This method 
of measurement gave such excellent results that the conductivities of the whole of the annealed rods were 
re-determined after this paper had been printed, and the corrected results were inserted in the proof, 
wherever any sensible difference was found. Hence the unannealed results are less reliable. 
} The standard copper was supplied by Messrs. White & Co., of Glasgow, and the conductivity of the 
sample determined in Lord Kelvin’s Laboratory at the University of Glasgow. 
