134 ‘ REPORT—1862. 
the Committee. Dr. Esselbach next points out that the unit of resistance 
which he proposes differs very little from Dr. Siemens’s mercury unit, which 
he, like your Committee, considers a great advantage; and the difference is, 
indeed, less than he supposes. He also proposes to use Weber’s absolute unit 
for the unit of current—a suggestion entirely in accordance with the fore- 
going Report; and he further points out that this current will be of con- 
venient magnitude for practical purposes. He next approves of the sugges- 
tions of Sir Charles Bright and Mr. Latimer Clark with reference to nomen- 
clature and terminology. In the body of his letter he gives some valuable 
data with reference to the unit of quantity, which he defines in the same 
manner as your Committee. This result will be analysed in the Report which 
Professor W. Thomson and Mr. Fleeming Jenkin will make on the fresh de- 
termination of the absolute unit of resistance. 
The Committee attach high importance to this communication, showing 
as it does that a practical electrician had arrived at many of the very same 
conclusions as the Committee, quite independently and without consultation 
with any of the members. Dr. Esselbach has omitted to point out, what he 
no doubt was well aware of, that, if, as he suggests, two equal multiples of 
the absolute units of resistance and electromotive force are adopted, the 
practical unit of electromotive force, or Daniell’s cell, will, in a circuit of 
the practical unit of resistance, produce the unit current. 
Mr. Fleeming Jenkin was requested to furnish an historical summary of 
the various standards of resistance, but he has been unable to complete his 
Report in time for the present meeting. 
Professor Williamson and Dr. Matthiessen were requested to put together 
the facts regarding the composition of the various materials hitherto used for 
standards of resistance, and the physical changes they were likely to undergo. 
Wires of pure solid metals, columns of mercury, and wires of alloys have 
been used for the purpose. The Report of the above gentlemen is appended 
(C). Init they arrive at the following conclusions:— _ 
First, with reference to pure metals in a solid state, they consider that the 
preparation of those metals ina state of sufficient purity to ensure a constant 
specific resistance is exceedingly difficult, as is proved by the great discre- 
pancy in the relative conducting powers obtained by different observers. 
Electrotype copper is excepted from this remark. They also point out that 
the influence of annealing on the conducting powers of pure solid metals is 
very great, and would render their use for the purpose of reproducing a stand- 
ard very objectionable, inasmuch as it is impossible to ensure that any two 
wires shall be equally hard or soft. They observe that errors of the same 
kind might be caused by unseen cayities in the wires, and give examples of 
the actual occurrence of these cavities. They point out another objection to 
the use of pure solid metals as standards, in the fact that their resistance 
varies rapidly with a change of temperature, so that slight errors in a ther- 
mometer or its reading would materially affect the results of an experiment. 
Secondly, with reference to mercury, they show that it is comparatively 
easily purified, varies little in resistance with a change of temperature, and 
can undergo no change analogous to that caused by annealing; but that, on 
the other hand, measurements of its conducting power by different observers 
vary much, that the tube used cannot be kept full of mercury for any length 
of time, as it would become impure by partial amalgamation with the ter- 
minals, and that consequently each time a mercury standard is used it has, 
practically, to be remade. The accuracy with which different observers can 
reproduce mereury standards has not been determined, 
