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 foregoing Report; and he further points out that this current will be of convenient magnitude for practical purposes. He next approves of the suggestions of Sir Charles Bright and Mr. Latimer Clark with reference to nomenclature 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 determination 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). In it they arrive at the following conclusions:

First, with reference to pure metals in a solid state, they consider that the preparation of those metals in a state of sufficient purity to ensure a constant specific resistance is exceedingly difficult, as is proved by the great discrepancy 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 standard 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 cavities 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 thermometer 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 terminals, and that consequently each time a mercury standard is used it has, practically, to be remade. The accuracy with which different observers can reproduce mercury standards has not been determined.

Thirdly, with reference to alloys, they say that there is better evidence of the independent and accurate reproduction of a standard by a gold-silver alloy of certain proportions than by a pure solid metal or by mercury. They point out that annealing and changes of temperature have far less effect on alloys than on pure metals, and that consequently any want of homogeneity or any error in observing the temperature during an experiment is, with alloys, of little consequence, but that, on the other hand, the existence of cavities must be admitted as possible in all solid wires. They are of opinion that the permanence of jewellery affords strong ground for believing that a gold-silver alloy will be quite as permanent as any solid pure metal; and in the course of the Report they point out some curious facts showing that a great change in the molecular condition of some pure metals and alloys may occur without any proportional change in their conducting powers.

Finally, they recommend that practical experiments should be made independently by several gentlemen to determine whether mercury or the goldsilver alloy be really the better means of reproducing a standard.

The main resolution arrived at by the Committee, viz. that a material standard shall be adopted which at the temperature of 17° Cent. shall approximetre mate to 107. as far as present data allow, has been already fully seconds' explained. It was not arrived at until after several meetings had been held and the merits of the various proposals fully discussed.

This resolution was passed (unanimously) at a meeting when five out of the six members of the Committee were present.

It was at the same time resolved that provisional copies should be distributed at the present Meeting; but the circumstances have been already explained which have prevented this resolution from being carried into effect. It was thought desirable that an apparatus should be designed which could be recommended by the Committee for use in copying and multiplying the units to be issued, since it is certain that some of the glaring discrepancies in coils intended to agree must have been due to defective modes of adjustment. Mr. Fleeming Jenkin has consequently designed an apparatus for the purpose, of which a description is appended. Messrs. Elliott Brothers have kindly constructed a couple of these instruments, which were seen in action, at the Meeting of the Association, by members interested in this subject. The present Report was drawn up by Mr. Jenkin, and adopted at a meeting of the Committee on the 30th of September.

Appendix to Report on Standards of Electrical Resistance.

A. On the variation of the electrical resistance of alloys due to change of temperature, by Dr. Matthiessen, F.R.S.

B. On the electrical permanency of metals and alloys, by Dr. Matthiessen, F.R.S.

C. On the reproduction of electrical standards by chemical means, by Professor Williamson, F.R.S., and Dr. Matthiessen, F.R.S.

D. Professor Kirchhoff's letter.

E. Dr. Siemens's letter.

F. Dr. Esselbach's letter.

G. Circular addressed to foreign men of science.

H. Description of apparatus for copying and multiplying the units of resistance.

APPENDIX A.-On the Variation of the Electrical Resistance of Alloys due to Change of Temperature. By Dr. MATTHIESSEN, F.R.S.

It has been shown that the influence of temperature on the electric conducting power of the metals amounts to 29.3 per cent. on their conducting power between 0° and 100° C. : an exception to this law has been found in iron†, the conducting power of which decreases between those limits 38.2 per cent. It was, therefore, useless to try any of the other pure metals, as they would, in all probability, have decreased by the same amount, as well as from the fact that the metals which would have suited the purpose had already been tried. I therefore turned my attention to the alloys, and, in conjunction with Dr. C. Vogt, have made a long series of experiments respecting the influence of temperature on their electric conducting power. After having determined the conducting power of a few of them at different temperatures, together with the help of the few experiments which have already been made by different observers, it became obvious that the percentage decrement in their conducting power stands in some relation to the fact that, when a solid metal is alloyed with another (with the exception of lead, tin, zinc, and cadmium amongst each other), a lower conducting power is observed than the mean of that of the components. The law which we found to regulate this property was with most alloys the following, viz. :—

"The percentage decrement between 0° and 100° in the conducting power of an alloy in a solid state stands in the same ratio to the mean percentage decrement of the components between 0° and 100° as the conducting power of the alloy at 100° does to the mean conducting power of the components at 100°;" or, in other words, "the absolute difference in the observed resistance between 0° and 100° of an alloy is equal to the absolute difference between the means of the resistance of the component metals between 0° and 100°."

For example, the conducting power of the hard-drawn gold-silver alloy was found equal to 15.03 at 0° (taking silver equal 100° at 0°), and decreases 6.49 per cent. between 0° and 100°. The mean decrement of the components between 0° and 100° being 29.3 per cent., the conducting power of the alloy is 14.05 at 100°, and that of the mean of the components is 62.58 at 100°. If we now calculate the percentage decrement in the conducting power of the alloy between 0° and 100° from the above data, we find it equal to 6:58 per cent., and by experiments it was found equal to 6-49 per cent. Or, taking the resistance of silver at 0°=100, and that of gold at 0°=128.3, we find the resistance of the alloy at 0°=665·3, and at 100°=711·7, and that calculated from a mean of the volumes of its components at 0°=113·2, and at 100°=159.8; therefore the absolute difference between the observed resistance at 0° and 100° is 46.4, and that between the calculated at 0° and 100°-46.8.

Knowing already, from my experiments on the electric conducting power of alloys§, that when two metals are alloyed together in any proportion, if the alloy is merely a solution of the two metals in one another, its conducting power may be approximatively foretold, and that, from the above law, it is necessary that if the conducting power of an alloy should vary between the limits of 0° and 100° to a minimum extent, the alloy itself must have a minimum conducting power as compared with that calculated from its *Phil. Trans. 1862, pt. 1.

+ Matthiessen and Vogt, unpublished researches.

Assuming that the conducting-power or resistance of an alloy is equal to that of parallel wires of the components forming it.

§ Phil. Trans. 1860, p. 161.

components,-I at once foresaw that it would be useless, as was afterwards proved by the research made in conjunction with Dr. Vogt, to make any experiments with the two metal-alloys, which may be looked upon as a solution of one metal in the other, as no practical alloy would be found which would vary in its conducting power between 0° and 100° to a small extent. It must also be borne in mind that the alloy sought for must be a ductile one, capable of being drawn into wire, not too soft, as would easily be damaged by covering and winding, easily produced, and cheap in price. Bearing this in mind, we turned our attention to some three metalalloys, thinking that we had some chance there of obtaining a good result; for it is well known that the conducting power of German-silver wire varies in such a slight extent between 0° and 100°.

It also appeared worth while to experiment with some of those alloys which may perhaps be considered chemical combinations, or to contain such, as, for instance, platinum and silver; and, on account of their other physical properties, the platinum-iridium alloys were also experimented with.

In the following Table I give the results obtained in conjunction with Dr. Vogt. The unit here taken for comparison is that of a hard-drawn silver wire at 0°. The normal wires were made of German silver, and in order to obtain their values in terms of hard-drawn silver, they were compared with the gold-silver alloy. In these experiments it was thought better first to use those pure metals which are easily obtained, so as to learn something regarding the manner in which the three metal-alloys behave, and then try some alloys made of the cheaper commercial metals. As will be seen by the Table, only the first part has been as yet carried out.

TABLE.

(With each series, the formula deduced from the observations for the correction of the conducting power of the alloy for temperature is given, when λ is equal to the conducting power at the temperature t C.) Composition of alloy. Weight.

Length 532 mm. ; diameter 0.625 mm.

This alloy was taken as Karmarsch states it is the hardest and most elastic of all the gold-silver-copper alloys.

Length 341:5 mm. ; diameter 0.618 mm.

λ=10·6220-0·0056248t+0·0000009863ť2.

This alloy was tried as it corresponded to equal volumes of gold-copper and gold-silver, and these again correspond to an alloy possessing the lowest conducting power of any of those made of gold-copper or gold-silver,

X=44-472-0-0815251+0-0003240ť.

This alloy was taken to see the effect such a combination would have.

Length 244 mm. ; diameter 0.682 mm.

X=4.541-0-0029307t+0.000002724ť.

This alloy was tried as it possesses very great elasticity and does not become softer on annealing. On account of these properties, as well as its permanency in air (not oxidizing on its surface), it would serve exceedingly well for making springs and contacts for electric and telegraphic apparatus.

Length 381.5 mm. ; diameter 0.451 mm.

This and the following two alloys were taken as they probably contain chemical combinations.

X=18.045-0.013960t+0.00001183ť..

Length 169 mm. ; diameter 0.408 mm.

In the following Table I have given the results in such a manner that

they may be easily compared.