120 REPORT—1890. 
taken ; it would, however, amount to about ‘0002, judged by the correc- 
tion required to observations on II., when leaking at a similar rate. 
The results from II. are equally consistent among themselves, but all 
slightly greater than those from I. This would indicate that the correc- 
tion applied for the leak in II. is rather too large. 
The capacities given in the table are those found with a 5 seconds’ 
interval, by which time, as we have seen, the absorption on the mica 
condensers used is practically complete. We have already discussed the 
method of determining the instantaneous capacity, and a table of the 
corresponding values could easily be given. 
For our present purpose it is hardly necessary to do this, and indeed 
for many purposes for which condensers are employed a knowledge of 
the full capacity is more useful than one of the instantaneous one. In 
the last column the values of the capacities found by the commutator 
method are given; the differences in both cases amount to about 1°3 per 
cent. of the capacity. 
During the forthcoming year condenser IJ. will be again set up and 
tested, and the permanent arrangements for rapidly comparing condensers 
and for issuing certificates will, I hope, be completed. 
APPENDIX IIT. 
On the Specific Resistance of Copper. By T. C. Frrzparricx. 
All the values given in tables for the specific resistance of the metals 
are directly or indirectly obtained from the values given by Matthiessen 
in his series of papers published in the ‘Transactions of the Royal 
Society’ for the years 1860-1864, and in the Reports of the British AssG=~» : 
ciation for the same years. " 
In the ‘ Transactions’! for the year 1860 is a paper by Matthiessen 
on the conductivity of pure copper, and on the effects of impurities on it; 
no alloy of copper having as high a conductivity as the pure metal. His 
results are expressed in terms of the conductivity of a hard-drawn silver 
ey es at ses Sati ig: following values for samples of copper 
carefully prepared by himself :— 
3 ee 2 ate Giving a mean value of 
(3) 93-02 af 18°°4,| 93°08 at 18°-9 as the 
(4) 92-76 ” 199.3 conductivity of pure 
5 jo. # o.k copper. 
(eo) 92:99) 55 (lv? 
Numbers are given showing the effect on the conductivity of small 
quantities of oxide, and he states that he found it necessary to pass 
hydrogen through the molten metal for some time for entire reduction. 
In the ‘Transactions’ for 1862 Dr. Matthiessen has a paper on the 
influence of temperature on the conductivity of metals. He again 
expresses his results in terms of a hard-drawn silver wire. On page 8 
of that paper will be found the results of his experiments on copper: 
the lowest temperature at which measurements were made was 12° or 
1 Phil. Trans. 1860, p. 85. 
