ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 121 
16°; he there shows how the results for pure copper measured at 18° 
may be reduced to 0° C.; but no measurement was actually made at 0° 
_ for any of the metals experimented with. 
He expresses the influence of temperature on a hard-drawn copper 
wire, the mean result of a number of determinations, by the equation 
A=100—-38701¢ + 00090092? 
where 100 is the conductivity of copper at 0° C., so that a hard-drawn 
silver and copper wire have the same conductivity at 0° C. 
The values obtained by comparison with a hard-drawn silver wire are 
then largely the source of the tables of specific resistances; but at the 
end of his appendix to the Report of the Electrical Standards Committee 
for 1864, Matthiessen gives values for hard-drawn silver and copper 
_ wires in terms of the new B.A. unit, expressed as the resistance of a wire 
_ one metre long, weighing one gramme. 
These values are :— 
Copper. ‘ ‘ F : . °1469 
Silver F ‘ ; A ‘ ; 1682 
The same table of values is given in the ‘ Philosophical Magazine’ for 
1865, where also is givena table of specific resistances for wires one metre 
long and one millimetre diameter, expressed in terms of the B.A. unit, 
and calculated from the value of the known conducting power of gold- 
silver alloy in terms of hard-drawn silver, and also in terms of the B.A. 
unit. 
The values thus obtained do not agree at all well with the results 
calculated for the resistances of the gramme metre by the specific gravi- 
ties of the elements furnished by tables. 
Thus :— 
Calculated Observed 
Silver. hs 02048 ; . ‘02108 
Copper . : "02090 d . 02104 
Matthiessen states that he omitted to determine the specific gravity 
of the copper used in his experiments; he probably would not have 
obtained any very accurate results, as the weight of copper he used 
varied from 1:5 to 4 grammes. 
The accuracy of Matthiessen’s results seems to depend, therefore, on 
the accuracy of his determination of the resistance in terms of the B.A. 
unit of a hard-drawn silver wire; in considering, therefore, the question 
of the preparation of samples of copper of higher conductivities than 
Matthiessen obtained, it may be suggested that the cause of the difference 
18 not explained by tbe fact that Matthiessen did not prepare pure copper, 
but by an error in the value of the standard with which the comparison 
aS made, 
I have, therefore, made a series of experiments on the resistance of 
@ silver wires; and, as a general result, have obtained a value iden- 
tical with that of Matthiessen ; the difference is not due, therefore, to an 
error in the standard employed, as far as my experiments go. 
___ Matthiessen does not give anywhere the details of his measurements 
of the specific resistances of the metals in terms of the B.A. unit; in the 
B.A. Report he simply mentions that an approximate table is subjoined, 
not even stating the fact that the values are for a temperature of 0° C. 
o conclude, therefore, that these values are caleulated out from the former, 
