72 Barrett, Brown & Haprretp—On the Electrical Conductivity and 
The copper standard was in the form of wire (0:124 em. diameter), whilst 
the iron standard and the alloys to be tested were in the form of rods (0°55 em. 
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. Tor 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 wire 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 wire. 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 cron, 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 o’ of the alloy in 
microhms per c.c.t In the latter case it is only necessary to multiply the 
conductivity of the specimen in terms of copper by 100, and divide by 16:86, 
or, in the case of the annealed specimens, divide by 16°8. Taking, for example, 
the last specimen named in the table on the opposite page, 1892 G, which in the 
annealed state has a conductivity of 9°8, copper being 100, we get 
9-8 x 100 
Sa es 
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. 
+ The reason for this is that the specific resistance o of the standard copper is 1°721 microhms at the 
1-721 x 100 
/ 
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o! 
/ 
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2 : c 
temperature at which the experiments were made, and as — = Fi ; 
