126 REPORT—1862. 
the first point were decided by a resolution in favour of a unit based on 
Professor Weber’s or Sir Charles Bright and Mr. Latimer Clark’s system, this 
decision would not affect the question of construction; while, on the other 
hand, if the second question were decided in favour of any particular arrange- 
ment of mercury or gold wire as the best form of standard, this choice would 
not affect the question of what the absolute magnitude of the unit was to be. 
The Committee have arrived at a provisional conclusion as to the first 
question ; and the arguments by which they have been guided in coming to 
this decision will form the chief subject of the present Report. : 
They have formed no opinion as to the second question, or the best form 
and material for the standard. 
In determining what would be the most convenient unit for all purposes, 
both practical and purely scientific, the Committee were of opinion that the 
unit chosen should combine, as far as was possible, the five following qualities. 
1, The magnitude of the unit should be such as would lend itself to the 
more usual electrical measurements, without requiring the use of extravagantly 
high numbers of ciphers or of long series of decimals. 
2. The unit should bear a definite relation to units which may be adopted 
for the measurement of electrical quantity, currents, and electromotive force ; 
or, in other words, it should form part of a complete system for electrical 
measurements, 
3. The unit of resistance, in common with the other units of the system, 
should, so far as is possible, bear a definite relation to the unit of work, the 
great connecting link between all physical measurements. 
4, The unit should be perfectly definite, and should not be liable to require 
correction or alteration from time to time. 
5. The unit should be reproducible with exactitude, in order that, if the 
original standard were injured, it might be replaced, and also in order that 
obseryers who may be unable to obtain copies of the standard may be able to 
manufacture one for themselves without serious error. 
The Committee were also of opinion that the unit should be based on the 
French metrical system, rather than on that now used in this country. 
Fortunately no very long use can be pleaded in fayour of any of the units 
of electrical resistance hitherto proposed, and the Committee were therefore 
at liberty to judge of each proposal by its inherent merits only; and they 
believe that, by the plan which they propose for adoption, a unit will be 
obtained combining to a great extent the five qualities enumerated as desi- 
rable, although they cannot yet say with certainty how far the fourth quality, 
of absolute permanency, can be ensured. 
The question of the most convenient magnitude was decided by reference 
to those units which have already found some acceptance, These, omitting 
metre 
onds 
copper wire weighing one hundred grains (a unit proposed by Professor 
Wheatstone in 1843) and one mile of copper wire of j;th in. in diameter, 
and weighing consequently about 844 grains per foot. The smaller units 
had generally been used by purely scientific observers, and the larger by 
engineers or practical electricians. 
Intermediate between the two lay Dr. Werner Siemens’s mercury unit, and 
the unit adopted by Professor W. Thomson as approximately equal to one 
hundred millions of absolute foot 
seconds 
for the moment Weber’s , were found to range between one foot of 
. The former is approximately equal to 
