122 REPORT—1863. 
any measurement yet made of the relative values of heat and work—a mea- 
surement corresponding to a great extent in its nature with that undertaken by 
the Committee. Nevertheless, considering the discrepancy of the various 
independent results, the Committee are of opinion that it is essential that 
the results of the Sub-Committee should be checked by a fresh series of expe- 
riments with a new coil in a distinct place, when every separate measure- 
ment will necessarily be repeated. The Sub-Committee especially urge the 
repetition of the experiments, as with the improvements already enume- 
rated, and other minor alterations, they confidently expect a considerably closer 
approximation to the absolute unit than they have hitherto obtained. It will be 
well here to remark that, according to the resolution of the Committee of 1861, 
the coils, when issued, will not be called absolute units, but the units of the 
British Association; so that any subsequent improvement in experimental 
absolute measurement will not entail a. change in the standard, but only a 
trifling correction in those calculations which involve the correlation of the 
physical forces. 
It is now time to leave the question of absolute measurement and pass to 
some of the other points under the consideration of the Committee. Dr. 
Matthiessen has, by careful experiment, proved the permanence for a year at 
least of the electrical resistance of certain wires; but he has detected a change 
in others, due, apparently, to the influence of time. Certain specimens of sil- 
ver, gold, and copper have varied; but other specimens of the same metals 
have remained constant. All the specimens of platinum and gold-silver alloy 
have remained constant, and all the specimens of German silver have changed 
considerably. It is proposed to continue and extend these experiments, and 
it is much to be hoped that the defect observed in the German silver tested 
will not be found common to all the varieties of this alloy, in other respects so 
well adapted for the construction of resistance-coils. Dr. Matthiessen found 
no difference in the resistance of wires of any of the above metals before and 
after the passage of a powerful current transmitted through them continually 
for a fortnight. The details of these experiments are given in Appendix A. 
Dr. Matthiessen has also continued his experiments with the object of find- 
ing an alloy with a minimum variation of resistance due to change of tem- 
perature, but has been unable to produce a wire superior in this respect 
to the silver-platinum alloy mentioned in Appendix A. of the Report of 
last year, as decreasing in conducting power 3-1 per cent. between 100° and 
0° Centigrade. German silver was found to decrease under the same circum- 
stances 4:4 per cent. 
The valuable experiments by Mr. Sabine, for Dr. Werner Siemens of Berlin, 
on the reproduction of standards by means of mercury, although not under- 
taken for the Committee, yet bear so directly on the subject before them that 
the results cannot be allowed to pass unmentioned. Dr. Siemens has con- 
clusively proved that he can, in his laboratory, reproduce a standard by means 
of mercury with an error of less than 0-05 per cent. This admirable result, 
while it seriously affects the question of the best material for the construction 
and reproduction of the standard, leaves, of course, the question of the best 
magnitude for the standard quite untouched. Dr. Matthiessen thinks that 
several of the solid metals are equally fitted for the purposes of reproduc- 
tion, and, if aided by the Association, is disposed to put his conviction to 
experimental proof. It is especially desirable that the various methods pro- 
posed should be tested by the concordance of the results obtained from a 
number of independent observers. 
With reference to the construction of the material standard, it is proposed 
