96 REPORT—1890. 
The change in F was referred to at the end of the Report in 1888, and 
is now very large. The coil has increased in resistance by abont ‘0006 
B.A. unit; G, on the other hand, has fallen by about ‘0002 B.A. unit, 
and H by about ‘0001 unit. The evidence for these various statements is 
given in an appendix to the Report by the Secretary. 
It is perhaps worth remark that in each case the change either took 
place during the time that the coil was immersed in ice or was found to 
have happened when the coil was next measured after its removal from 
the ice. 
The legal ohm coils have not varied relative to Flat. 
The investigations into the resistance of copper have been continued 
by Mr. Fitzpatrick. The Committee desire again to thank the gentlemen 
who have rendered him assistance. 
Mr. Fitzpatrick has examined various specimens of copper supplied 
him as wire. He has also examined copper prepared for him as pure by 
Messrs. Sutton, as well as some which he prepared himself electrolytically 
from carefally purified copper sulphate. These last two specimens lead 
to practically the same value as that obtained by Matthiessen for the 
specific resistance of copper—-viz., 1767 x10-® B.A. units at 18°; the 
specific gravity of these specimens is about 8°90. Two wires supplied to 
him have, however, a distinctly lower resistance : the value for one being 
1731 x10-°, and for the other 1724x10~°; a difference in the one case of 
2 and in the other of 2°4 per cent. The specific gravity of the first of 
these wires is 8940 and of the other 8:946, and Mr. Fitzpatrick assigns 
the increased conductivity to increased density rather than to greater 
urity. 
; Matthiessen gives his results for the resistance of copper at 0°. The 
observations were, however, made mostly at a temperature of 18° or 20°, 
and reduced to 0° by the use of a temperature coefficient ; so that the 
value at 18° found from that at 0° by the same coefficient will probably 
vepresent the result of Matthiessen’s work more accurately than the one 
he gives himself. Various other points of importance are discussed in 
Mr. Fitzpatrick’s appendix. He hopes to be able to give the results for 
some copper prepared by chemical 
means by Mr. Skinner and himself. 
He has also made a number of mea- 
surements on silver, but these are not 
yet complete. 
Dr. Muirhead and the Secretary 
have both been working indepen- 
dently at the construction and mea- 
surement of a standard air condenser. 
Two such condensers have been 
made for the Committee by the Cam- 
bridge Scientific Instrument Com- 
pany, on a plan suggested by Dr. 
Muirhead, and meutioned in the last 
report. The capacity of each of these 
is about ‘02 microfarad. Some slight 
alterations are required to one of 
FERS these, the other is completely satis- 
factcry. Its capacity has been repeatedly found, and remains constant 
to at least within 1 in 2,000, which is about the limit of accuracy 
Fig. 1. 
acseescuse==-=-> 
Ae 
