72 Barrett, Bijown & Hadfield — On the Electrical Conductivity and 



The copper standard was in the form of wire (0-124 cm. diameter), whilst 

 the iron standard and the alloys to be tested were in the form of rods (0-55 cm. 

 diameter). In order to compare the copper and iron standards in the same 

 physical condition, and also to check the comparison of their conductivity just 

 given, a careful comparison was made of the specific resistance of the copper and 

 of that of the iron. For this purpose a sample of the standard iron was drawn 

 into wire of about the same diameter as the standard copper : a determination 

 of its specific resistance gave 10-47 microhms per c.c. at 18° C. The specific 

 resistance of the copper was found to be 1-721 microhms per c.c. at 18° C* 

 The ratio of the reciprocals of these two numbers give the conductivity of iron 

 as 16-36, copper being taken as 100. 



The standard iron tvire was now compared directly with the standard copper 



wire by the potential method, and the result gave a conductivity of 16-37 for 



iron, copper being 100; practically the same result as when the standard iron 



rod was compared with the standard copper tvire. The mean of these three 



results gives a value of 



16-36 



for the conductivity of our standard iron, Matthiessen's copper being taken as 

 100 and at the same temperature. 



The foregoing experiments also enable us (1) to obtain a factor for the 

 conversion of the conductivities of the various alloys given in the tables below 

 into specific resistances in microhms per c.c. ; and (2) to give the conductivities 

 of the alloys in terms of the standard iron, taken as 100. In the former case 

 all that is necessary is to divide 172-1 by the conductivity of the specimen as 

 compared with copper, and the result is the specific resistance cr' of the alloy in 

 microhms per c.c.f In the latter case it is only necessary to multijily the 

 conductivity of the specimen in terms of copper by 100, and divide by 16-36, 

 or, in the case of the annealed s^BcvaxQus,, divide by 16-8. Taking, for example, 

 the last specimen named in the table on the opposite page, 1392 G, which in the 

 annealed state has a conductivity of 9-8, copper being 100, we get 



9-8 X 100 _ 



16-8 - ^^•'^'^' 



which gives its conductivity, taking the standard iron as 100. 



* The resistance of both the iron and the copper here given is somewhat higher than usually stated in 

 the table of physical constants, even at 18° C; this is due to the fact that] both the copper and iron 

 in the above test -were unannealed. The resistance of the copper was determined in Lord Kelvin's 

 laboratory as well as by ourselves. 



fThe reason for this is that the specific resistance u of the standard copper is 1-721 microhms at the 



, . , , . , (7 c' , 1-721 X 100 

 temperature at which the experiments were made, and as — , = — o- = -^ • 



