348 



REPORT — 1864. 



must be constructed of various materials, aud protected in various ways, for 

 reference and comparison. 



The precautions taken to prevent chemical action and mechanical injury- 

 are given in Appendix B. of the Report for 1SG3. Coils of wire covered with 

 silk, baked, and imbedded in solid paraffin, appear, at present, to be the 

 most promising form for the unit standards. Authentic copies of the standard 

 coils made of platinum-silver aUoj-, which appears likely to be permanent, 

 might be issued at about £2 10s. each, and coils prepared from these by 

 electrical instrument-makers could be verified at a moderate rate at Kew, 

 where the original standards will be deposited. No officially authentic 

 . coil can be issued uutil the standards themselves have been made. 



The reproduction of the standard forms the next point for consideration. 

 NotwithstancUng the good results obtained by Professor Thomson's method of 

 making an absolute measurement, the Sub-Committee do not recommend the 

 adoption of this process for the reproduction of the standard, which may 

 some day become necessary, owing- to the accidental destruction of, or change 

 in the Kew standards. Dr. Matthiessen, on the other hand, states, with con- 

 fidence, that a standard may be reproduced by means of metal wires of given 

 weight aud length, or by means of mercury, within about 0-01 per cent. ; the 

 report of his investigation on this subject, made conjointly with Mr. C. Hoc- 

 kin, is contained in Appendix C, and may be summed up as follows. He first 

 draws a distinction between ordinary care, great care, and absolute care. He 

 considers that with ordinary care the gold-silver alloy is the most suitable 

 material (see Eeport, 18G2) for the reproduction ; but when great care is used 

 lead is recommended as the most suitable material, but any reproduction bj^ one 

 material should be checked by others, such as mercury. With absolute care 

 it appears that almost any material might be used. It must be remembered 

 that Dr. Matthiessen considers that he himself has not taken absolute but 

 only great care. 



The following Table shows the number of wires of each material tested, 

 their maximum discrepancy, and the probable error in a standard reproduced 

 by similar experiments : — 



Commercially pure lead differed from the chemically pm'e lead by only 

 about 0-04 per cent. 



Por an account of the care taken by Dr. Matthiessen in the chemical pre- 

 paration of the metals he used, and in their subsequent treatment and 

 electrical comparison, we must refer to Dr. Matthiessen's own Eeport, Ap- 

 pendix C. 



With reference to mercury, great difficult}' exists in making the experi- 

 ments, and it is much to be regretted that Dr. Matthiessen's experiments, very 

 accordant in themselves, do not give results agreeing with Dr. Siemens's 



