ON STANDARDS OF ELECTRICAL RESISTANCE. 151 
introduction of one unit of electrical resistance. I gladly respond to the 
invitation to express my view on the manner in which the desired object 
might be best attained. 
To define the unit of resistance by the resistance of a wire of given 
dimensions of a pure metal appears to me impossible, for the reasons which 
have been urged by the Committee; hence, of the three proposals discussed 
by the Committee, there only remain two for our consideration. 
1. To adopt the unit proposed by Weber; or, 2. To establish, as unit of 
resistance, the resistance of a column of pure mercury of given dimensions 
and at a given temperature. 
I do not think that to these a third of equal value can be added; for te 
define the unit of resistance by the thermal action of an electrical current 
would certainly never answer the purpose, because this thermal action 
cannot be measured with the necessary accuracy, and the resistance of any 
wire which is to be permanently kept cannot be fixed as unit; for the 
resistance of any wire for a given temperature certainly undergoes changes if 
electrical currents are transmitted through it, and it is exposed to fluctuations 
of temperature. 
Of the above two units, the first recommends itself by coming up more 
satisfactorily to the demands of science; the second, as I think, by being 
capable for the present of being practically carried out with greater accuracy. 
But is it really necessary to decide for one and against the other of these two 
units? I think not. If the ratio between them is established with the accu- 
racy which is now attainable, there can, I think, arise no more confusion from 
their simultaneous use, than from the practice of expressing lengths sometimes 
in metres and sometimes in millimetres. You say, “It is proposed that the 
unit adopted shall be represented by one particular standard, constructed of 
very permanent materials, laid up in a national repository ;” and further, 
“The Committee will probably endeavour to devise some plan by which 
copies of the actual material standard adopted may be easily procured at a 
reasonable cost.” This plan, the execution of which I consider highly 
desirable, might evidently be realized in all its essential points without its 
being necessary to give the preference to one of these units over the other: 
it would only be necessary to measure the resistance of the normal standard 
in both units, and to add to each copy its resistance expressed in both units, 
In choosing the metal or the alloy of which the normal standard and the 
copies are to be made, care must undoubtedly first be taken that the 
resistance is as unalterable as possible for one temperature. It is undoubtedly 
desirable that the resistance shall not vary rapidly with the temperature. 
This is, however, not very important, provided that the temperature of the 
wire can be accurately observed at any moment. To satisfy this condition, 
the wires must not be coiled upon cylinders, but fastened so that, for the 
greater part of their extent, they lie clear, and hence rapidly assume the 
temperature of the surrounding air or of the non-conducting liquid in which 
they may have been immersed. 
You request me to point out to you any researches of mine which refer to 
a unit of electrical resistance. Ihave to mention a short treatise only, which 
appeared in vol. lxxvi. of Poggendorff’s ‘ Annalen,’ under the title “ Deter- 
mination of the Constants on which the Intensity of Induced Electrical Currents 
depends,” and which formed the answer to an academical prize-question which 
Professor Neumann, in Kénigsberg, had proposed in the year 1846. In this 
treatise a unit of electrical resistauce has not been suggested; but in it the 
resistance of a wire has been measured by the unit (or rather by double the 
