482 Royal Society : — 



a standard by the Electric and International Company, and about 

 once and a half Jacobi's unit*. 



It was found necessary to undertake entirely fresh experiments in 

 order to determine the actual value of the abstract standard, and to 

 express, the same in a material standard which might form the basis of 

 sets of resistance-coils to be used in the usual manner. These expe- 

 riments, made during two years with two distinct sets of apparatus by 

 Professor J. C. Maxwell and the writer, according to a plan devised 

 by Professor W. Thomson, are fully described in the Reports to the 

 British Association for 1863 and 1864. 



The results of the two series of experiments made in the two years 

 agree within 0*2 per cent., and they show that the new standard does 

 not probably differ from true absolute measure by 0*1 per centf. It 

 is not far from the mean of a somewhat widely differing series of deter- 

 minations by Weber. 



In order to avoid the inconvenience of a fluctuating standard, it is 

 proposed that the new standard shall not be called "absolute mea- 



sure," or described as so many- — — , but that it shall receive a 



seconds 



distinctive name, such as the B. A. unit, or, as Mr. Latimer Clark 

 suggests, the " Ohmad," so that, if hereafter improved methods of 

 determination in absolute measure are discovered or better experi- 

 ments made, the standard need not be changed, but a small coefficient 

 of correction applied in those cases in which it is necessary to convert 

 the B. A. measure into absolute measure. Every unit in popular use 

 has a distinctive name ; we say feet or grains, not units of length or 

 units of weight ; and it is in this way only that ambiguity can be 

 avoided. There are many absolute measures, according as the foot 

 and grain, the millimetre and milligramme, the metre and gramme, 

 &c. are used as the basis of the system. Another chance of error 

 arises from the possibility of a mistake in the decimal multiple used 

 as standard. For all these reasons, as well as for convenience of 

 expression, the writer would be glad if Mr. Clark's proposal were 

 adopted and the unit called an Ohmad. 



Experiments have been made for the Committee by Dr. Mat- 

 thiessen, to determine how far the permanency of material standards 

 may be relied on, and under what conditions wires unaltered in di- 

 mension, in chemical composition, or in temperature change their 

 resistance. Dr. Matthiessen has established that in some metals a 

 partial annealing, diminishing their resistance, does take place, ap- 

 parently due to age only. Other metals exhibit no alteration of 

 this kind ; and no permanent change due to the passage of voltaic 

 currents has been detected in any wires of any metal — a conclusion 

 contrary to a belief which has very generally prevailed. 



The standard obtained has been expressed in platinum, in a gold- 



* This last number may be 30 per cent, wrong, as the writer has never been in 

 possession of an authenticated Jacobi standard, and has only arrived at a rough 

 idea of its value by a series of published values which afford an indirect com- 

 parison. 



t Vide Appendix B. 



