Mr. R. Appleyard on the Conductometer. 281 
and a very uncertain quantity is thus eliminated from his 
calculations. Dr. Wilson concludes from his experiments 
that “it may be considered established that e lies between 
2x10-%° and 4x10-'° ELS. units.” The lower limit is in 
fair agreement with the value 1:2 10-! found for H by 
taking N=10”. We see, therefore, that the value 2 x 10~”° 
does "nok differ by more than the (eee 2 from the most 
ie values which can be obtained by both methods. 

XXVIII. The Conductometer. By Rotto APPLEYARD*. 
HIS is a direct-reading instrument, intended for the 
comparison of electrical conductivityt of copper and 
other wires, for a range within, say, 5 per cent. above and 
) per cent. below 100. 
In comparing two wires, either may be regarded as the 
standard. Suppose that balance is obtained with two samples 
of equal length upon a straight bridge-wire, divided into 
100 parts, the position of balance being L divisions: (A) as- 
suming the two wires to be of the same mean diameter, 
but of different conductivities; (B) assuming the wires to 
be of equal conductivity, but of unequal diameters. In 
case (A) it is found from the conditions of balance that a 
change of 1 per cent. conductivity between +5 Foe cent. and 
—5 per cent. corresponds on the average with zl, of the 
total length of the bridge-wire. If therefore the middle of 
the bridge-\ wire is marked “ 100 ” and divisions are ay ked off 
from that point to right and left, each equal to 71, of the 
length of the bridge-wire, these approximately correspond to 
successive increments of 1 per cent. conductivity. Or again, 
if the standard wire is not 100 per cent., move the whole 
scale thus graduated so that the mark on it corresponding 
to the conductivity of the standard is in coincidence with the 
electrical middle of the bridge-wire. I have proved that this 
arrangement is still direct-reading, and that its indications 
may be trusted to within a considerable degree of accuracy. 
In case (B), suppose the two wires have diameters d and 
(d+y) respectively. Then an expression for L in terms 
of y and d can be found. If in this expression y is given 
some definite value, say 1 mil, and if d is then given suc- 
cessive values corresponding to the whole range of diameters 
of wires in common use, a table or curve can “be constructed 
showing at once the amount by which the slider must be 

* Communicated by the-Physical Society: read December 11, 1903, 
+ See “The Electrical Conductivity of Copper,” Electrical "Revi lew, 
June 19, July 3 & 10, and August 14, 1903. 
