366 REPORT — 1864. 



ment wifh nitric acid. The chief labour is in selecting and calibrating the 

 tubes, and this is vei-y great. 



The results obtained with the gold-silver aUoy, even when prepared by 

 different persons, supposing great care used, give very accordant results, 

 and for the easiness with which it can be made may be recommended for re- 

 producing an unit. 



■ Electrotype copper would appear a good siibstance. The agreement of 

 results obtained with commercial electrotype copper with those obtained 

 with copper prepared from pure salts shows this. 



The maximum difference of the conducting-powers of electrotype copper, 

 as observed with ordinary care, is 1-6 per cent. Copper is not, however, to 

 be preferred, as great care and some practice is necessary to draw a good 

 wire. 



The pirrification and drawing of pure gold and silver would, in the hands 

 of anyone but a chemist, lead to no good results, iu aU probability. These 

 facts being considered, we should prefer lead for the reproduction of an unit. 

 No doubt it would be well to use two or three substances to check the results. 

 For these auxiliary substances mercury and the gold-silver alloy may be re- 

 commended. The choice between these two will depend on the appliances of 

 the indi\ddual observer. When thorough care is taken aU the above means 

 are equally good. 



On forming an opinion on the difficulty of reproducing an unit by chemical 

 means it must be remembered that if anything like .accuracy is wished for, 

 not only expensive and delicate apparatus is required, but also very much time 

 must be spent, and a great deal of experience in the manipulation is required. 

 The experiments here described extended over about six months. Any 

 person wishing to reproduce an unit should bear these considerations in mind, 

 especially as it is the intention of your Committee to cause coils to be issued 

 representing a known resistance. That copies of a given resistance can be 

 made to a much greater acciu'aey than that to be obtained by chemical or 

 other known means of reproduction, and that coils can be compared by dif- 

 ferent observers witli different apparatus to almost any degree of accuracy, 

 although this fact has been brought into question by a former experimenter, 

 is proved by the following facts. 



The two units whic'h have come into our hands, made by Messrs. Siemens 

 and Halske from copies of the coil used last year by your Committee for the 

 determination of the absolute unit, were compared against the standard coU 

 and found to agree with it within two-hundredths per cent. Again, copies of 

 Weber's unit, one made of the gold-silver alloy, the other of German silver, 

 were compared at the interval of two years by different observers with dif- 

 ferent apparatus, and the results found to agree to one -half a hundi-eth per 

 cent. 



It is from the fact that copies can be produced with almost absolute 

 accuracy, with a minimum of cost and labour as compared with chemical or 

 mechanical means of reproduction, that we seem quite justified in recom- 

 mending all who wish to obtain a standard to procure a copy of the British 

 Association unit, or any other in general use. As copies of the British Asso- 

 ciation unit are being sold at a reasonable price by several of the leading in- 

 strument-makers, which we are given to understand wiU agree together very 

 closely, we confidently recommend the adoption of this unit. 



And in conclusion, we still adhere to the opinion, given in Appendix C of 

 the Report of 1862, that the best means of reproducing an unit, for those 

 who have not the opportunity of procuring a copy, and who cannot afford the 



