ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 129 
perature of his observation, supposing the samples of copper of the same 
character. 
Matthiessen’s results are given in terms of a gramme per metre, and 
for wires of metre length and 1 mm. in diameter. 
In a paper in the ‘ Philosophical Magazine,’ Matthiessen gives the 
value for hard-drawn copper in these terms as 
02104 B.A. 
From his value for the gramme metre, using the specific gravity 8°95 
given by tables, the same quantity was calculated, but gave the result 
0209 ; in a note added he states that had he used the specific gravity 8°91 
his results would have been more nearly alike; but a specific gravity 8:90, 
I find, would give an identical value. 
This would show, then, that Matthiessen’s own table, calculated for 
values obtained by comparison with hard-drawn silver, is accurate. I 
have tested silver wires, but have not had time to draw up the results in 
tabular form ; and I obtained an almost identical value for hard-drawn 
silver wire, as supplied me from Messrs. Johnson & Matthey, as is given 
by Matthiessen for the resistance of a gramme per metre. 
It will be observed that wires IX. have the specific gravity 8°90, and 
give a value in terms of B.A. units for a cubic centimetre of the material 
identical with Matthiessen’s value; this value is not given directly by 
Matthiessen, but is calculated from his results by Fleeming Jenkin, and 
given in his table in his book ‘ Electricity and Magnetism’: it is 1-652 
microhms. I have calculated it from Matthiessen’s value, given in the 
‘ Philosophical Magazine,’ and get the number 1653. Using the same 
temperature coefficient as before, the resistance at 18° C. of a cubic 
centimetre of hard-drawn copper is 17666 x 10-° B.A. units. 
On comparing the values for wires [X., X., and XI. in these terms, 
the results do not agree so well together as when expressed in terms of 
the gramme metre ; there is a corresponding difference in the values of 
the specific gravities ; these latter have been very carefully determined, 
and the experiments repeated with the results given. 
Wires, therefore, of the same resistance expressed for grammes per 
metre, may give a very different result, when expressed as per cubic 
centimetre : attention has been drawn to this fact in the discussion on 
the Elmore copper in the ‘ Electrician.’! M. Roux, of Paris, in a letter 
gives the following table for high-conductivity wire from a paper of 
M. Hospitalier in ‘L’Electricien,’ 1887; this paper I have unfortu- 
nately not been able to see. 
Density . : : ‘ 8:897 9°32 9°6 
Conductivity, equal volume 102-4 106°7 110°8 
Conductivity, equal weight 101-7 101°2 101°6 
What is 100 in the conductivity units is not expressed. M. Roux 
thinks that the former, i.e., for equal volume, is the more rational method 
of expressing the result. 
Matthiessen expressed all his results in terms of equal weight, justify- 
ing it by the greater accuracy obtainable when working with small weights 
of wires. Smallerrors in the value of the specific gravity are easily 
made, and cause a similar error in the result for equal volumes of 
1 Hlectrician, Decemker 7, 1888. 
1890. ° K 
