70 Barrett, Brown & Haprietp— On the Electrical Conductivity and 
specimen of Swedish charcoal iron, marked 8.C.I.) was made with the following 
result :— ; 
Carbon, . : : . 0:028 per cent. 
Silicon, . : , .  0:070 5 
Sulphur, . . : : . 0:005 5p 
Phosphorus, ‘ : 5 . 0:044 ‘3 
Manganese, : : = (@itrace)) |; 
Tron, ; : 5 : . 99°85 (by difference). 
This standard iron rod was accurately turned to a uniform diameter through- 
out. Its dimensions were 104 ems. long, and 0:4889 cm. diameter, and it 
contained, as seen above, less than 0:03 per cent. of carbon. The electrical 
arrangements, shown diagrammatically in fig. 1, were as follows :— 
Fi@. 1. 
The standard rod S, and the rod to be tested, 7, were put in series with a 
battery B, a variable resistance #, and a Weston ampére-meter & reading to one- 
hundredths of an ampére ; when the plug key, A, was inserted, a constant current 
could be maintained through S and 7 by altering R. Flexible leads were taken 
from a sensitive high resistance dead-beat reflecting galvanometer V, to two 
copper knife-edges JM and JV, which acted as contact-pieces to the rods. The fall 
of potential over a definite known length of each rod was thus obtained on 
reading the deflection given by the potential galvanometer V. Contacts were 
first made with the standard S, then with the test 7, and again with the standard 
(as a check on the constancy of the current), and the deflection in each case 
noted. The fall of potential over one-half and over one-quarter of the rod under 
test was also taken and compared with the standard; by this means the 
homogeneity and uniformity of sectional area in the rods were tested, as the deflec- 
tions in these cases should be one-half and one quarter of that given by the whole 
rod. ‘The diameter of each of the rods under examination was taken in six 
