353 REPORT — 1864. 



All the values have been reduced to the first observed temperature, assuming 

 that all pure metals vary in conducting-power alike with temperature. The 

 correction made was the addition or subtraction of 0'036 for each 0°-l, 

 which number corresjionds to the correction of conducting-power for tempe- 

 rature at 20°. No correction has been made in the cases of No. 15 and 16, 

 for it is so small that it may be neglected, being about 0-006 for each 0°4. 



As stated in last year's Report, the differences may be considered due to 

 temperature ; for, as there explained, a difference in the temperature of the 

 wire and the bath might well exist, and we find in most cases a difference 

 in the conducting-power corresponding to 0°'l to 0°-2. 



It is interesting to find that hard-drawn silver and copper wires become 

 partially annealed by age, at least the increment in the conducting-power 

 would indicate such to be the case. In the case of silver, a decided increment 

 wiU be observed. 



No. 8, copper, annealed, has altered so much, that there can be no doubt 

 that it was badly soldered. 



With regard to the alteration observed with the German-silver wires, it 

 may here again be stated that it is not to be assumed that all wires of this 

 alloy will alter in like manner. An example of this has lately come to my 

 notice. About two years ago I made a coil of the gold-silver alloy, which 

 was compared with one of Prof. Thomson's German-silver coils, and having 

 them stiU in my possession, they have now been re-compared ■svith the follow- 

 ing results: — 



July 8th, 18C2. Eesistanee of Thomson's coil being 1 at 18°-4, that of 

 the gold-silver coil was -88445 at 18°-4. 



August 6, 1864. Eesistanee of Thomson's coil being 1 at lS°-4, that of 

 the gold-silver coil was -88447 at 18°-4. 



It is worthy of remark that the first comparison was made bj' Dr. C. Vogt, 

 the last by Mr. C. Hockin, and with entirely different apparatus, showing 

 that different observers with different apparatus obtain absolutely the same 

 results, when they take great care in making the observations. 



The above proves that the conducting power of aU specimens of German- 

 silver wire does not alter bj^ age. Further experiments are being made on 

 this subject, and in the course of a year or so we shall be able to say how 

 far German silver may be trusted for making resistance coils. 



Appendix C. — On tJie Beproduction of Electrical Stmulards hi/ Chemical Means. 

 By A. Matthiessen, F.E.S., and C. Hockin, Felloiu of St. Jolm's College, 

 Cambridfje. 



Having been requested by your Committee to make some experiments with 

 the view of discovering the best method of reproducing an unit of electrical 

 resistance by chemical means, we have carried out the research of which we 

 now propose to give the results. 



The experiments have been made with unusual care. It is important to 

 point out the degree of precaution that has been taken to insure trustworthy 

 results. The care taken in these experiments may be called great care as 

 opposed to ordinary care on the one hand, and thorough care on the other. 

 By ordinary care is meant the care usually taken in scientific research, where 

 no extraordinary precautions are had recourse to. The sort of accuracy 

 obtained when a unit is reproduced with ordinary care may be seen by 

 reference to former results. For instance, in the determination of the con- 

 ducting-power of mercury, described in ' Phil. Trans.', results were obtained 



