RESISTANCE OF TUNGSTEN 
231 
making six determinations for the given point. Six such sets 
of six readings each were made along the wire, one set for 
each of six different points. The mean of these observations was 
found and the probable error of the mean calculated. This 
process was repeated for each of the ten wires. Rotation of the 
wire is necessary since the cross section of a wire is seldom, if 
ever, a circle. The relatively large values of the probable er- 
rors of the diameter measurements are due to this same fact. 
The specific resistance of the wires was next determined. Each 
end of a wire was held in a clamp made of a piece of one-quarter 
inch brass rod which could be fastened to the terminals of the 
Wheatstone’s Bridge or Kelvin Double Bridge as the case might 
be. The use of the clamps made it possible to measure the re- 
sistance of the same length of wire by both methods. Also the 
length of the wire was determined by measuring the distance 
between the faces of the damps, when the wire was held 
stretched, by means of a pair of dividers and a steel scale. Each 
length used in the calculations is the mean of three measure- 
ments. The resistances as found by the Kelvin Double Bridge, 
w T ith one exception, were used in the computations, since this; 
method gives a gyeater accuracy. The percentage probable er- 
rors of the resistance and length measurements are far less than 
those of the radii and were therefore neglected in computing 
the probable errors of the specific resistances. The results are 
given in table 1. 
TABLE 1. 
SET 1. 
No. of 
wire 
Length 
cm. 
Radius 
cm. x 10 3 
Resistance 
Specific 
Resistance 
ohms/cm 3 x 10® 
Wheatstone | 
Kelvin 
A 
19.10 
2.396±0.003 
6.507 
6.142±0.019 
B 
32.16 
7.836± .007 
1.018 
L018 
6.106± .0111 
C — _ 
29.21 
12.61 ± .015 
0.3480 
0.3474 
5.940± .013; 
D _ __ 
33.81 
17.61 ± .018 
0.2043 
0.2046 
5.894± .012: 
E 
37.55 
22.70 ± .025 
0.1342 
0.1339 
5.763± .013; 
SET 2. 
A' _ , 
31.86 
5.086±0.006 
2.381 
2.404 
6.128±0.014- 
B' 
33.84 
10.33 ± .015 
0.5980 
0.6051 
5.990± .017 
C' 
34.31 
15.03 ± .009 
0.2872 
0.2867 
5.930± .007 
D' 
39.32 
21.90 ± .075 
0.1550 
0.1546 
5.909± .041 
E' 
37.27 
25.84 ± .023 
0.1050 
0.1050 
5.900± .011 
