128 REPORT—1890. 
Below is a table of the measurements made for the determination of 
their specific resistances :— 
Length of 
Weight wire for | Length cut Resis 
Wire Value of Temp. of determina- and Hees 
1/3 5 ° 5 & P 
wire tion of re- | weighed metre 
sistance. 
TX. (1) "28547 i Ufo) 20°388 192-1 192°5 1574 TSS:3 
ay C2) “28541 17 Oc 20°153 192°4 190°45 1569 ar (5 
X. (1) *28550 18°2 19°708 189°3 188°8 1577 18°-6 
52) *28536 16°°8 20°252 192°39 192-34 1561 aha 
XI. *28535 16%7 20°262 192-11 192°51 1563 LOD 
These values reduced to a common temperature of 18° are :— 
1X, '() : : "1572 
IX. (2) 3 , 1572 Mean value 
Rha : : 1573 1571 
X. (2) ; ‘ 1569 B.A. unit. 
XI. : s 1569 
Thus ‘1571 B.A. unit is the resistance at 18° of a metre of hard- 
drawn copper wire weighing 1 gramme. 
Matthiessen in the B.A. Report ' gives as the resistance of a gramme 
metre at 0° :1469 B.A. unit. 
T have calculated from this the value at 18°, using the temperature 
coefficient that he gives in his paper on the influence of temperature on 
the conducting power of metals. I have taken no account of the terms 
in ¢? as they practically cancel one another. 
R. 18° = R° (1 + *0038701¢). 
Ros = alba 
This is the value that I have obtained as the mean of my own 
observations. 
All my observations were taken at the temperature of the room, and 
in the table above the values for the different wires are given at the 
observed temperature, and then all reduced to a common temperature of 
18° C. Most observations of this character are taken at the temperature 
of 0° C., but on the whole it seemed more satisfactory to work at the 
temperature of the room. In the comparison of the B.A. units I have 
found that with a difference of temperature between coils which are 
connected by thick pieces of copper there is always conduction of heat, 
and it is impossible to tell accurately what is the real temperature of the 
coils. 
My observations were made in the B.A. room at the Cavendish 
Laboratory, which has a north aspect, and often the temperature did 
not alter more than a few tenths of a degree, whilst the temperature of 
the coil baths often remained perfectly steady for several consecutive days. 
T cannot find any observations of Matthiessen’s at 0° C.; certainly his 
observations on copper were made at 18°, and, consequently, if the value 
given by him at 0° C. has been obtained by the use of a temperature co- 
efficient, my value might be expected to agree with his at 18°, the tem- 
B.A. Teport, 1864, or Phil. Mag. 1865. 
