130 REPORT—1862. 
metre 
seconds 
error resulting from the use of the 1862 unit in dynamical calculations instead 
of the true absolute unit may be corrected by those who require these correc- 
tions, but that the material standard itself shall under no circumstances be 
altered in substance or definition. 
By this plan the first condition is fulfilled; for the absolute magnitude of 
this standard will differ by only 2 or 3 per cent. from Dr. Siemens’s mercury 
standard. 
The second and third conditions will be fulfilled with such accuracy as 
science at any time will allow. 
The fourth condition, of permanency, will be ensured so far as our know- 
ledge of the electrical qualities of matter will permit; and even the fifth 
condition, referring to the reproduction, is rendered comparatively easy of 
accomplishment. 
There are two reasons for desiring that a standard should be reproducible: 
first, in order that if the original be lost or destroyed it may be replaced ; 
secondly, in order that men unable to obtain copies of the true standard may 
approximately produce standards of their own. It is indeed hoped that accurate 
copies of the proposed material standard will soon be everywhere obtainable, 
and that a man will no more think of producing his own standard than of 
deducing his foot rule from a pendulum, or his metre from an are of the 
meridian ; and it wil! be one of the duties of the Committee to facilitate the 
obtaining of such copies, which can be made with a thousandfold greater 
accuracy than could be ensured by any of the methods of reproduction 
hitherto proposed. 
It is also hoped that no reproduction of the original standard may ever be 
necessary. Nevertheless great stress has been lately laid upon this quality, 
and two methods of reproduction have been described by Dr. Werner Siemens 
and Dr. Matthiessen respectively ; the former uses mercury, and the latter an 
alloy of gold and silver, for the purpose. Both methods seem susceptible of 
considerable accuracy. The Committee have not yet decided which of the two 
is preferable ; but their merits have been discussed from a chemical point of 
view in the appended Report C, by Prof. Williamson and Dr. Matthiessen. 
An interesting letter from Dr. Siemens on the same point will also be found 
in the Appendix E. This gentleman there advocates the use of a metre of 
mercury of one square millimetre section at 0° C. as the resistance unit ; but his 
arguments seem really to bear only on the use of mercury in constructing 
and reproducing the standard, and would apply as well to any length and 
section as to those which he has chosen. 
When the material 1862 standard has once been made, whether of platinum, 
gold and alloy, or mercury, or otherwise, the exact dimensions of a column of 
mercury, or of a wire of gold-silver alloy, corresponding to that standard can 
be ascertained, published, and used where absolutely necessary for the pur- 
pose of reproduction. 
It should at the same time be well understood that, whether this reproduc- 
tion does or does not agree with the original standard, the unit is to be that 
one original material permanent standard, and no other whatever, and also 
that a certified copy will always be infinitely preferable to any reproduction. 
The reproduction by means of a fresh determination of the absolute unit 
would never be attempted, inasmuch as it would be costly, difficult, and 
uncertain; but, as already mentioned, the difference between new absolute 
should be ascertained with increased accuracy, in order that the 
