158 REPORT—1862. 
3rd. That their use would be a simple extension of the system already 
universally adopted in magnetic measurements. 
4th. That the unit is independent of the physical properties of any material. 
Against the system it is urged that the unit cannot be determined with 
sufficient accuracy, and that even its approximate reproduction, where copies 
cannot be obtained, is difficult and expensive. 
In favour of the mercury standard the following arguments are used :— 
1st. No change can occur in the molecular structure or temper of the 
material, and therefore the same tube filled with pure mercury will certainly 
always conduct alike. 
2nd. Change of temperature causes only a slight difference in resistance. 
Against this plan it is said— 
1st. That tubes cannot be made of uniform or similar wires, and that, 
therefore, the standard once broken is lost for ever. 
2nd. That the standard tube cannot be kept full of pure mercury, owing to 
the admixture which would take place of the solid metal used for the terminals, 
so that each time the standard has to be used it has practically to be remade. 
3rd. That the attempt, by most observers, to reproduce the unit for their 
own use would be attended with incorrect results, as is shown by the different 
results obtained by different observers. 
In favour of Dr. Matthiessen’s alloy, as compared with wires of pure metal, 
or with mercury, as a material for the standard, it is said— 
Ist. That the variations of resistance, corresponding with variations of 
temperature or temper, are small. 
2nd. That a unit expressed in this material can be more readily and 
certainly reproduced than one expressed by a pure metal, because the 
presence of slight impurities in the component metals, or a slight change 
in their proportion, does not sensibly affect the result. 
Against this plan it is said that the physical properties of an alloy are 
more likely to change than those of a pure metal. 
Against all the plans for standards, based on an arbitrary length and 
section of an arbitrary material, the supporters of the absolute units state 
that the adoption of such an arbitrary standard would lead to great con- 
fusion and complication in the measurement of all other electrical properties, 
and in the expression of the relation of such measurements to those of force, 
work, heat, &e. 
This objection does not, of course, apply to the expression of the absolute 
unit by means of a wire of pure metal, of an alloy, or by mercury: but it is 
urged that no observer should ever attempt the reproduction of a standard 
when a copy of the proposed universal standard can possibly be obtained ; 
and the Committee will probably endeavour to devise some plan by which 
such copies of the actual material standard adopted may be easily procured at 
a reasonable cost. 
It will be seen from the resolutions passed, that the Committee are now 
engaged in investigating the degree of accuracy with which Weber’s units can 
be obtained, and the degree of permanency which may be expected from the 
use of the metal or alloy forming the material standard expressing these or 
other units. 
The Committee will feel greatly indebted to you if you will afford them the 
benefit of your valuable advice and experience on the above points, and on any 
others which may occur to you. They also venture to Hope that such a standard 
may be selected as will give very general satisfaction ; and, if approved by you, 
that you will kindly take an interest in procuring its general adoption. 
