_Nature_ states that the Queen's Printers are now issuing the Report
(dated July 23, 1891) to the President of the Board of Trade, of the
Committee appointed to consider the question of constructing standards
for the measurement of electricity. The committee included Mr.
Courtenay Boyle, C.B., Major P. Cardew, R.E., Mr. E. Graves, Mr. W.H.
Preece, F.R.S., Sir W. Thomson, F.R.S., Lord Rayleigh, F.R.S., Prof.
G. Carey Foster, F.R.S., Mr. R.T. Glazebrook, F.R. S., Dr. John
Hopkinson, F.R.S., Prof. W.E. Ayrton, F.R.S.
In response to an invitation, the following gentlemen attended and
gave evidence: On behalf of the Association of Chambers of Commerce,
Mr. Thomas Parker and Mr. Hugh Erat Harrison; on behalf of the London
Council, Prof. Silvanus Thompson; on behalf of the London Chamber of
Commerce, Mr. R. E. Crompton. The Committee were indebted to Dr. J.A.
Fleming and Dr. A. Muirhead for valuable information and assistance;
and they state that they had the advantage of the experience and
advice of Mr. H. J. Chaney, the Superintendent of Weights and
Measures. The Secretary to the Committee was Sir T.W. P. Blomefield,
Bart.
The following are the resolutions of the Committee:
_Resolutions._
(1) That it is desirable that new denominations of standards for the
measurement of electricity should be made and approved by Her Majesty
in Council as Board of Trade standards.
(2) That the magnitudes of these standards should be determined on the
electro-magnetic system of measurement with reference to the
centimeter as unit of length, the gramme as unit of mass, and the
second as unit of time, and that by the terms centimeter and gramme
are meant the standards of those denominations deposited with the
Board of Trade.
(3) That the standard of electrical resistance should be denominated
the ohm, and should have the value 1,000,000,000 in terms of the
centimeter and second.
(4) That the resistance offered to an unvarying electric current by a
column of mercury of a constant cross sectional area of 1 square
millimeter, and of a length of 106.3 centimeters at the temperature of
melting ice may be adopted as 1 ohm.
(5) That the value of the standard of resistance constructed by a
committee of the British Association for the Advancement of Science in
the years 1863 and 1864, and known as the British Association unit,
may be taken as 0.9866 of the ohm.
(6) That a material standard, constructed in solid metal, and verified
by comparison with the British Association unit, should be adopted as
the standard ohm.
(7) That for the purpose of replacing the standard, if lost,
destroyed, or damaged, and for ordinary use, a limited number of
copies should be constructed, which should be periodically compared
with the standard ohm and with the British Association unit.
(8) That resistances constructed in solid metal should be adopted as
Board of Trade standards for multiples and sub-multiples of the ohm.
(9) That the standard of electrical current should be denominated the
ampere, and should have the value one-tenth (0.1) in terms of the
centimeter, gramme, and second.
(10) That an unvarying current which, when passed through a solution
of nitrate of silver in water, in accordance with the specification
attached to this report, deposits silver at the rate of 0.001118 of a
gramme per second, may be taken as a current of 1 ampere.
(11) That an alternating current of 1 ampere shall mean a current such
that the square root of the time-average of the square of its strength
at each instant in amperes is unity.
(12) That instruments constructed on the principle of the balance, in
which, by the proper disposition of the conductors, forces of
attraction and repulsion are produced, which depend upon the amount of
current passing, and are balanced by known weights, should be adopted
as the Board of Trade standards for the measurement of current,
whether unvarying or alternating.
(13) That the standard of electrical pressure should be denominated
the volt, being the pressure which, if steadily applied to a conductor
whose resistance is 1 ohm, will produce a current of 1 ampere.
(14) That the electrical pressure at a temperature of 62° F. between
the poles or electrodes of the voltaic cell known as Clark's cell may
be taken as not differing from a pressure of 1.433 volts by more than
an amount which will be determined by a sub-committee appointed to
investigate the question, who will prepare a specification for the
construction and use of the cell.
(15) That an alternating pressure of 1 volt shall mean a pressure such
that the square root of the time average of the square of its value at
each instant in volts is unity.
(16) That instruments constructed on the principle of Sir W. Thomson's
quadrant electrometer used idiostatically, and for high pressure
instruments on the principle of the balance, electrostatic forces
being balanced against a known weight, should be adopted as Board of
Trade standards for the measurement of pressure, whether unvarying or
alternating.
We have adopted the system of electrical units originally defined by
the British Association for the Advancement of Science, and we have
found in its recent researches, as well as in the deliberations of the
International Congress on Electrical Units, held in Paris, valuable
guidance for determining the exact magnitudes of the several units of
electrical measurement, as well as for the verification of the
material standards.
We have stated the relation between the proposed standard ohm and the
unit of resistance originally determined by the British Association,
and have also stated its relation to the mercurial standard adopted by
the International Conference.
We find that considerations of practical importance make it
undesirable to adopt a mercurial standard; we have, therefore,
preferred to adopt a material standard constructed in solid metal.
It appears to us to be necessary that in transactions between buyer
and seller, a legal character should henceforth be assigned to the
units of electrical measurement now suggested; and with this view,
that the issue of an Order in Council should be recommended, under the
Weights and Measures Act, in the form annexed to this report.
_Specification referred to in Resolution 10._
In the following specification the term silver voltameter means the
arrangement of apparatus by means of which an electric current is
passed through a solution of nitrate of silver in water. The silver
voltameter measures the total electrical quantity which has passed
during the time of the experiment, and by noting this time the time
average of the current, or if the current has been kept constant, the
current itself, can be deduced.
In employing the silver voltameter to measure currents of about 1
ampere, the following arrangements should be adopted. The kathode on
which the silver is to be deposited should take the form of a platinum
bowl not less than 10 cm. in diameter, and from 4 to 5 cm. in depth.
The anode should be a plate of pure silver some 30 square cm. in area
and 2 or 3 millimeters in thickness.
This is supported horizontally in the liquid near the top of the
solution by a platinum wire passed through holes in the plate at
opposite corners. To prevent the disintegrated silver which is formed
on the anode from falling on to the kathode, the anode should be
wrapped round with pure filter paper, secured at the back with sealing
wax.
The liquid should consist of a neutral solution of pure silver
nitrate, containing about 15 parts by weight of the nitrate to 85
parts of water.
The resistance of the voltameter changes somewhat as the current
passes. To prevent these changes having too great an effect on the
current, some resistance besides that of the voltameter should be
inserted in the circuit. The total metallic resistance of the circuit
should not be less than 10 ohms.
_Method of making a Measurement._--The platinum bowl is washed with
nitric acid and distilled water, dried by heat, and then left to cool
in a desiccator. When thoroughly dry, it is weighed carefully.
It is nearly filled with the solution, and connected to the rest of
the circuit by being placed on a clean copper support, to which a
binding screw is attached. This copper support must be insulated.
The anode is then immersed in the solution, so as to be well covered
by it, and supported in that position; the connections to the rest of
the circuit are made.
Contact is made at the key, noting the time of contact. The current is
allowed to pass for not less than half an hour, and the time at which
contact is broken is observed. Care must be taken that the clock used
is keeping correct time during this interval.
The solution is now removed from the bowl, and the deposit is washed
with distilled water and left to soak for at least six hours. It is
then rinsed successively with distilled water and absolute alcohol,
and dried in a hot-air bath at a temperature of about 160° C. After
cooling in a desiccator, it is weighed again. The gain in weight gives
the silver deposited.
To find the current in amperes, this weight, expressed in grammes,
must be divided by the number of seconds during which the current has
been passed, and by 0.001118.
The result will be the time average of the current, if during the
interval the current has varied.
In determining by this method the constant of an instrument the
current should be kept as nearly constant as possible, and the
readings of the instrument taken at frequent observed intervals of
time. These observations give a curve from which the reading
corresponding to the mean current (time average of the current) can be
found. The current, as calculated by the voltameter, corresponds to
this reading.
* * * * *