70 



Barrett, Brown & Hadfield — On the Electrical Conductimty and 



specimen of Swedish charcoal iron, marked S.C.I.) was made with the following 

 result : — 



This standard iron rod was accurately turned to a uniform diameter through- 

 out. Its dimensions were 104 cms. 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: — 



Fia. 1. 



The standard rod S, and the rod to be tested, T, were put in series with a 

 battery B^ a variable resistance R^ and a Weston ampfere-meter Q reading to one- 

 hundredth s of an ampere ; when the plug key, ff, was inserted, a constant current 

 could be maintained through 8 and T by altering R. Flexible leads were taken 

 from a sensitive high resistance dead-beat reflecting galvanometer F, to two 

 copper knife-edges M and iV, 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 jootential galvanometer V. Contacts were 

 first made with the standard /S*, then with the test T^ and again with the standard 

 (as a check on the constancy of the current), and tlie 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 



