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trodes of considerable surface might be employed; but I believe that the removal of the copper connexions after each test, and the removal of the old mercury from their surfaces before using them again, are a sufficient safeguard against error arising from this source. Besides, it is easy to fill the spiral with fresh mercury whenever it is suspected to have dissolved any quantity of copper, or even on every occasion when a measurement with it is to be made. Nor does mercury change its resistance in the least by standing in the air. This I have proved by keeping a spiral six months filled without changing the mercury, and found its resistance to be constant.

The material which I have extensively employed in copying this measure, viz. German silver, may be classed under the same head as the expensive gold-silver alloy of Dr. A. Matthiessen, over which it has, however, the considerable advantages of a greater specific resistance, and that its resistance varies less with temperature variations.

As a preventive against alteration of resistance by the influence of the air, I have usually had the resistances made of this metal covered with a coating of silk and lac.

Intermediate between the resistances to be measured and the measure itself, I have introduced resistance-scales. These contain each a series of resistances (multiples of the unit), and are so arranged that each resistance is exact when it stands stopped alone in the circuit. When carefully made, these scales may be depended on to 0.1 per cent.

Being convinced of the sufficiency of the method I have described of reproducing a standard of electrical resistance, I have the honour to suggest to you, 1st. To recommend the universal adoption of the conducting power of mercury as unit, and of the resistance which a prism of that metal, a metre long, and square millimetre section, at 0° C., opposes to a current of electricity as common unit of resistance.

2nd. To have the value of this measure ascertained, with the greatest possible exactness, in absolute units.

3rd. To have copies of this unit constructed in mercury contained in glass spirals for preservation in scientific repositories.

In the event of my suggestions being adopted, the mercury unit should be determined again with the greatest possible care, and with all the help which pure and applied science offers, and copies of it made with equal exactness.

According to a late determination by Weber, the mercury unit is only about 2 per cent. greater than 1010 absolute units, or one mercury unit at-26° C. would equal 10,000,000,000 absolute units.

Since those cases in which the expression of resistances in absolute measure is of advantage in facilitating calculations occur only very seldom, and only in purely scientific exercises, a single determination of the relation of the two measures would be amply sufficient. Should the absolute unit or any multiple of it be adopted as common unit of resistance, there would still be wanted a unit for expressing the conducting powers of bodies; and mercury is indisputably the best calculated for this purpose. And for practical purposes, which in adopting a universal unit should be principally taken into consideration, it is indispensable to define the resistance-measure as a geometrical body of that material which is selected as unit of conducting power. Every other definition would not only burden unnecessarily the calculations which occur in common life, but also confuse our conception of the measure. The reason why the arbitrary unit proposed by Jacobi (a length of copper only approximately defined) found no admittance into general use is to be sought in the fact that it failed to fulfil this condition, and because the con

ducting power of all solid bodies is too dependent on their molecular struc

ture.

The same objection renders the adoption of the gold-silver alloy proposed by Dr. A. Matthiessen equally incapable.

Another disadvantage in the way of a solid metal unit is the impossibility to solder thick connexions into the ends of a defined length of any wire without altering its resistance.

Should the adoption of the mercury unit be deemed advisable, I would place at the service of the British Association any further information or assistance in my power.

I have the honour to be, Gentlemen,
Your most obedient Servant,

W. SIEMENS.

APPENDIX F.-Extracts from a Letter addressed to Professor WILLIAMSON by Dr. ESSELBACH.

The two objections against the practical applications of Weber's absolute unit have been sufficiently pointed out as being—

1. Its minuteness; and

2. That the electromotive force of galvanic elements does not allow of variation (as strength of current, tension, and resistance do), but that we have to accept certain constants as nature has fixed them.

I take it for granted that the standard of absolute unit would not lose in authority if a plain multiple of it were adopted. I need not point out that the French metre itself is only a submultiple, 10,000,000th of a natural unit—the earth's quadrant. The multiple of the natural electro-magnetic unit I am about to suggest for practical use is 1010, therefore very simple (which is of no little importance); and it is a multiple which leads us to those standards which are practically used.

M. Bosscha gives the electromotive force of his Daniell's cells in absolute

measure as

1025.80.10,

and calculates the one used by Mr. Joule to be

1045.1. 10.

It will therefore be practicable to determine such concentration of sulphuric acid as to make the electromotive force equal to

10. 1010;

and I believe the concentration required would be very near what is actually used in telegraphy.

Resistance. The different copies of Jacobi's étalons are well known to differ as much between each other as Daniell's cells; and if Siemens had done nothing else for galvanometry than to give us copies which agree among themselves within a quarter per cent., the progress is obvious.

Weber's copy of Jacobi's étalon is

and that of M. Bosscha was

in absolute measure.

598. 107;

607.107

Other statements (of Kirchhoff and others) give a much smaller value. In comparing Mr. Siemens's mercury standard with three copies of Jacobi's étalon in his possession, I found two of them agreeing tolerably well with

each other, and with a third one copied by my friend Dr. Teddersen, at Leipzig, from the original of M. Leyser, which I took therefore to be the more correct ones. I found the absolute value of Siemens's unit to be

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We should therefore only have to multiply all observations expressed in 1010 Siemens's units by to reduce them to absolute measure, and the suggested multiple for the future standard would not be far from 1.1 of Siemens's units, which every one admits to be for metallic conductors a practical unit.

1.1

For the resistance of insulating materials the figures become impracticably high; but it would be a matter of professional telegraphy to adopt, in conformity with the system, the resistance' 10" and, besides, another great resistance' containing 1010 resistances.'

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While the resistance of a mile of copper in an ordinary cable would be (say) 4 R. (four resistances), the insulation-resistance of a mile of cable would be about 0.04 G. R. (great or gutta-percha resistances).

My suggestion would therefore be

1. To adopt Weber's absolute unit, and to derive from it, by the multiple 1010 (or 10,000,000,000), the practical unit.

2. To adopt 1010 of Weber's electro-magnetic units as the 'practical absolute unit' for electromotive force and resistance.

(10 of these units would be exactly 1 Daniell's cell.)

3. 1 of these units would be 1·1 of Siemens's units.

6

4. To allow, besides, a practical great unit,' viz. 1010 of the 'practical units,' for resistances in order to express the insulation-resistance of cables in convenient figures.

5. To allow also a practical small unit' of

1 1010

absolute units to express

insulation-currents and charge-quantities of cables in convenient figures.

6. To adopt, in order to avoid confusion, for such practical units' a terminology as proposed by Messrs. Bright and Clark.

London, September 18, 1862.

APPENDIX G.-Circular addressed to Foreign Men of Science.

SIR,-I am requested to inform you that a Committee was appointed by the British Association, which met last year at Manchester, to report on Electrical Standards of Resistance.

The Committee consists of the following gentlemen :

Professor A. W. Williamson, F.R.S. | Professor W. H. Miller, F.R.S. (Cam(University College, London).

Professor Charles Wheatstone, F.R.S. (London).

Professor William Thomson, F.R.S. (Glasgow).

bridge).

A. Matthiessen, Ph.D., F.R.S. (London).

Fleeming Jenkin, Esq. (London).

The Committee met on December 6th, 1861, and on April 3rd, 1862. On the latter occasion the following Resolution was passed:

:

"Resolved, That the following gentlemen be informed of the appointment of the present Committee, and be requested to furnish suggestions in furtherance of its object.

Professor Edlund (Upsala).

Professor Th. Fechner (Leipzig).

Dr. Henry (Washington).
Professor Jacobi (St. Petersburg).
Professor G. Kirchhoff (Heidelberg).
Professor C. Matteucci (Turin).

Professor Neumann (Königsberg).
Professor J. C. Poggendorff (Berlin).
M. Pouillet (Paris).

Werner Siemens, Ph.D. (Berlin).
Professor W. G. Weber (Göttingen)."

I have, in consequence, the honour of addressing you the present letter. The Resolutions passed at the two meetings are enclosed, and from them you will gather the general scope of the Committee's inquiry. I add some further explanation as to the object and intentions of the Committee.

Great inconvenience has been felt from the absence of any generally adopted unit for the measurement of electrical resistance, and it was thought that the influence of the British Association might be successfully exerted to procure the adoption of a common standard. The present time was thought especially favourable, since, although the methods of observation have been brought to great perfection, no local units have as yet taken very deep root.

The units which up to the present time have been considered by the Committee may be classed under three heads :

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1st. A given length and weight or section of wire made of some pure metal, and observed at a given temperature, as originally proposed by Professors Wheatstone, Jacobi, and others.

2nd. Units based on Weber's and Gauss's system of absolute measure

ment.

3rd. A given length and section of pure mercury at a given temperature. Whatever basis is adopted for the unit, it is proposed that the unit adopted shall be represented by one particular standard, constructed of very permanent materials, laid up in a national repository; and it has been proposed to use Dr. A. Matthiessen's gold-and-silver alloy for this purpose. The arguments which have been used for and against these systems are as follows:In favour of the use of a wire of some pure metal it is said— That the plan is the simplest possible, and admits of independent observers forming their own standard.

Against the plan it is said

1st. That even when pure, two apparently similar wires do not resist equally unless their temper or molecular condition be the same—a condition which cannot practically be ensured.

2nd. That there is reason to believe that the resistance of a given wire is not constant even at a constant temperature.

3rd. That the resistance of all pure metals varies very rapidly with the temperature.

4th. That great difficulty is found in obtaining any metal pure, and that the attempt of most persons to reproduce the unit for their own use would be attended with incorrect results. This is evidenced by the different relative results as to the resistance of pure metals published by different observers. In favour of Weber's units it is urged

1st. That their use will ensure the adoption of a complete system of corresponding standards for electrical currents, quantities, and tension or difference of potential.

2nd. That their use is essential in the dynamic treatment of any problem connected with electricity; for instance, in determining the heat generated, the force exerted, the work done, and the chemical action required or produced under any given circumstances.

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—

1st. 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 confusion 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, &c.

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.

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