statement, which I have previously made, that it is impossible to prepare wires on the small scale which are of the same quality, i.e., probably due to density, as the best specimens specially prepared by large manufacturers.

In Table B are given the results of measurements on three specimens of copper prepared by Mr. Swan: one was given to Profs. Dewar and Fleming; a second was examined by Mr. Swan himself, and a third specimen he kindly sent to me; the quality of the copper in the three cases may therefore be expected to be the same.

The results of Profs. Dewar and Fleming and Messrs. Swan and Rhodin are expressed only as specific resistances, whilst the result of my measurement is only given for a wire one metre long weighing one gramme. The weight of the copper wire, as measured, was only three grammes, and that does not allow the accurate determination of the specific gravity of the sample. The value I obtain for its resistance is identical with that for the sample of annealed copper wire sent me by Messrs. Bolton.

If it be considered to have the same specific gravity as that sample (8.94) its specific resistance in C.G.S. units is 1665; a value distinctly smaller than that obtained by Messrs. Swan and Rhodin, whose result is practically identical with that of Profs. Dewar and Fleming.

Not only may wires drawn from the same specimen of copper have different densities and different resistances, but the variation of that resistance with change of temperature may be also different.

In the following table are given the temperature coefficients of various specimens of copper :

Influence of Annealing.-As is well known, annealed wires have a less resistance than hard-drawn wires, but the variation of resistance according as the wires are annealed or hard-drawn differs considerably for different materials. For silver it is as much as 10 per cent., whereas for copper it is less than 3 per cent.

I have made observations from time to time on the resistance value of specimens of hard-drawn copper wire, all pieces of the same coil, which were sent me in 1889 by Messrs. Bolton and Son. From the results of these measurements it will be seen that a hard-drawn wire seems to fall

in resistance with lapse of time. The coil of wire has been left hanging in the laboratory, and has not been treated with any special care.

B.A. Report, 1864.
Nature, vol. 1. p. 165.

2 Phil. Mag., vol. xxxvi. p. 287.
Electricity, vol. v. p. 165.

5 Comptes Rendus, lxxvi. p. 345.

The fall in resistance is small, and for the period of nearly five years does not amount to more than per cent.

I have, for the sake of comparison, made a measurement of the resistance of a specimen of annealed copper wire sent me by the same firm, and for this the resistance value is identical with that obtained at a previous date :

This, on the whole, is what one would expect. In the case of wires of other material the change would probably be greater, as the difference in resistance between annealed and hard-drawn copper wires is less than that for wires of other materials.

In my previous communication a method of annealing was described which gave satisfactory results. The wire was packed in asbestos and fine carbon in a copper vessel and heated for twenty-four hours. The following results amongst others were obtained :—

Messrs. Swan and Rhodin give for the values of the specific resist

ance:

Hard-drawn 18°
1720

Annealed 18°
1680

Difference

40

I have recently been annealing copper wires by heating them in boiling paraffin (220°); and after slow cooling the wires seem to be completely annealed :

Hard-drawn 18°
1526

Annealed 18°
1486

A wire sent me as annealed gave the result :

Annealed 18°, 1488

Difference
40

This wire was then hardened, and, reannealed as above described, gave the value:

Annealed 18°, 1489

B.A. Report, 1890, p. 126.

Either of these two methods seems to give satisfactory results. For completely annealing silver wires the temperature of the paraffin bath is not sufficiently high, but from the results of my measurements for silver, for which the influence of annealing is very considerable, it can be seen that the first method is quite satisfactory.

SILVER.

Many of the older measurements for resistances and conductivities are expressed in terms of the resistance of pure silver : this was the case with Matthiessen's earlier results.

Some measurements therefore made on silver wires are given, together with the results obtained by Matthiessen and Profs. Dewar and Fleming for the sake of comparison.

Several samples of silver wires were supplied by Messrs. Johnson and Matthey one of these was stated to be absolutely pure.

The results are expressed for wires weighing one gramme per metre.

The difference between the values for the hard-drawn wires is probably due to the fact that they had to be further drawn down after I had received them to enable me to measure them on my bridge.

Profs. Dewar 2 and Fleming give as the value for an annealed pure silver wire 1468 C.G.S. at 0° C. with the temperature coefficient of 004; the value at 18° is therefore for the specific resistance 1574.

For most of the wires which I measured the specific gravities were determined; for the wires Silver I. there is practically no difference between the values obtained for the annealed and hard-drawn wires, the values varying from 10.496 to 10.511.

For the wires Silver III. the values varied from 10-49 to 10.50; using the mean value 10.495 I get for the specific resistances the following values :

1

In the case of copper with increase of purity there is a decrease in the difference in resistance between annealed and hard-drawn wires. With silver the reverse is the case.

The value that I obtained for the hard-drawn wire is very nearly the same as that given by Matthiessen, but he obtained a greater decrease in resistance on annealing. He states 2 that for different pieces of the same wire there was a variation of from 6 to 10 per cent.; so that the difference between his value and that which I have obtained for a sample of pure silver is not greater than might be expected.

The considerable variation in all the values given above makes it clear that the values of the specific resistance depend not simply on the purity of the material, but on a number of other factors, which will be different in the cases of different wires of the same material, and that therefore we cannot expect to attain to any great degree of accuracy in the determination of specific resistances as distinguished from the accurate measurement for some particular wire.

APPENDIX VII.

Final Report of the Electrical Standards Committee of the Board of Trade. To the RIGHT HON. JAMES BRYCE, M.P.,

President of the Board of Trade.

Since the date of our last Report the Board of Trade have laid before us a résumé of the action of the International Electrical Congress held in Chicago in August 1893 to determine the units of electrical measurement. We were also informed by the Board of Trade that her Majesty's Government had been invited by the United States Ambassador in London to take steps to adopt the recommendations of the Congress.

These recommendations, so far as they refer to the units of electrical resistance, electrical current, and electrical pressure, are substantially the same as those suggested for adoption in our previous Reports.

We see no reason for further delay in the legalisation of standards of the above-mentioned units, and we have prepared and attach a revised Draft Order in Council, which we advise may be submitted for her Majesty's gracious approval.

The accompanying notes to the specification for the Clark's cell have been communicated by Mr. Glazebrook, and will be found of great assistance in the preparation of this form of cell.

The Order in Council is printed in the form in which it has since received her Majesty's approval.

For the notes see p. 141.

Order in Council regarding Standards for Electrical Measurements. At the Court at Oshorne House, Isle of Wight, August 23, 1894. Present: The Queen's Most Excellent Majesty in Council.

Whereas by 'The Weights and Measures Act, 1889,' it is among other things enacted that the Board of Trade shall from time to time cause such new denominations of standards for the measurement of electricity as appear to them to be required for use in trade to be made and duly verified.

And whereas it has been made to appear to the Board of Trade that new denominations of standards are required for use in trade based upon the following units of electrical measurement, viz.—

1. The ohm, which has the value 109 in terms of the centimetre and the second of time, and is represented by the resistance offered to an unvarying electric current by a column of mercury at the temperature of melting ice 14.4521 grammes in mass of a constant cross-sectional area and of a length of 106-3 centimetres.

ΤΟ

2. The ampere, which has the value in terms of the centimetre, the gramme, and the second of time, and which is represented by the unvarying electric current which when passed through a solution of nitrate of silver in water in accordance with the specification appended hereto, and marked A, deposits silver at the rate of 0.001118 of a gramme per second.

3. The volt, which has the value 108 in terms of the centimetre, the gramme, and the second of time, being the electrical pressure that if steadily applied to a conductor whose resistance is one ohm will produce a current of one ampere, and which is represented by 6974 (1994) of the electrical pressure at a temperature of 15° C. between the poles of the voltaic cell known as Clark's cell set up in accordance with the specification appended hereto, and marked B.

And whereas they have caused the said new denominations of standards to be made and duly verified.

Now, therefore, her Majesty, by virtue of the power vested in her by the said Act, by and with the advice of her Privy Council, is pleased to approve the several denominations of standards set forth in the schedule hereto as new denominations of standards for electrical measurement. C. L. PEEL.

Schedule.

I.-Standard of Electrical Resistance.

A standard of electrical resistance denominated one ohm being the resistance between the copper terminals of the instrument marked 'Board of Trade Ohm Standard Verified, 1894,' to the passage of an unvarying electrical current when the coil of insulated wire forming part of the aforesaid instrument and connected to the aforesaid terminals is in all parts at a temperature of 15°•4 C.

II. Standard of Electrical Current.

A standard of electrical current denominated one ampere being the current which is passing in and through the coils of wire forming part of the instrument marked 'Board of Trade Ampere Standard Verified, 1894,' when on reversing the current in the fixed coils the change in the forces