36 M. W. Siemens on a new reproducible Standard 
in constructing a standard of German silver wire. The resist- 
ance of spiral A at 0° was 514°45, that of B 673°9. 
German silver wire is very well adapted for standards of resist- 
ance, because its conducting power is very small, and varies little 
with the temperature; according to Arndtsen, only about ‘0004 
per cent. for a degree Centigrade. 
In the foregoing experiments, the resistance of a cube 1 millim. 
each way has been assumed as the unit. ‘or small resistances, and 
especially in calculation, this has many conveniences. It appears, 
however, more advisable that the measure of resistance should 
be in entire agreement with the ordinary metrical scale. I 
therefore determined to take as the unit of resistance,— 
The resistance of a prism of mercury 1 metre long and 1 millim. 
in section at the temperature O° C. 
If this proposal be generally acted on, all resistances can be 
at once reduced to the metrical seale. very experimenter will 
then be able to provide himself with a standard measure as accu- 
rate as his instruments permit or require, and to check the varia- 
tions of resistance of the more convenient metal standard. Of 
course also, in that case, the conducting power of mercury must 
be taken as the unit of conducting power, and not that of copper 
or silver. Unfortunately but few comparisons have been made 
between the conducting power of mercury and the solid metals, 
from which a table could be calculated in which the conducting 
power of mercury should be the unit; and in most of the com- 
parisons that have been made between the conducting power of 
the solid metals, it is not stated whether the wire employed was 
cold drawn or annealed. From the following Table, however, it 
appears that the conducting power of annealed wire is consider- 
ably greater than that of unannealed. 
a 2. 3. ° 4. 5. 6. 
Gaerne oe +, Weight in : : Conducting 
pecies of wire. Length in) ai. Specific Resistance! power, that 
millims, peace gravity. at 0° C. | of mercury 
TL Salver, hard ...5..0005: 4014:4 | 4884-9 | 10-479 | 614:55| 56-252 
Pe) BRNEDIED, ” chains. 4014°4 | 4889-1 | 10°492 537°2 64:38 
RS SC Ts Ge a ee -| 4014:4 | 3233-1 | 10502 896°1 58-20 
4° sa@nnealed Vin. 40144 | 8009°6 | 10:5132) 889-08} 63°31 
Se Coppers ard ss. ps0 oSss- 4014°4 | 3099-5 8-925 $90:5 527109 
A. MOTE coe. sajannees 4014-4 | 4409-1 8916 | 622-7 52-382 
” annealed ...... 4014-4 | 43855:2 89038 599-05 | 52-013 
5 ” BATE 3 cicccsesst 2007°2 | 1260-4 8-916 545'8 52°217 
” annealed ,..... 2007:2 | 1252-7 8-894 517 55°419 
eas |e ae ee RA 2007-2 | 1263-2 8 916 545°6 52:12] 
* annealed ...... 2007°2 | 1241°5 8-894 520°8 55°338 
7. Platinum, hard ......... 436-4 | 544-1 | 21-452 | 9106 8-244 
Bi. 354) hat ii. 436-4 | 550-1 | 21:452 897:7 8:27 
O..BPass; HAA o. yass ge sh: 1003°6 | 1406-1 8473 530°6 11-439 
»  annealed.........| 1003°6 | 1397°8 8:464 | 451:7 13:502 
