September 3, 1891] 



NATURE 



435 



by a column of mercury of a constant cross sectional area of 

 I square millimetre, and of a length of io6"3 centimetres at the 

 temperature of melting ice may be adopted as l 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 '9866 of the ohm. 



(6) That a material standard, constructed in solid metal, a-^d 

 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 de- 

 nominated the ampere, and should have the value one-tenth 

 (O'l) in terms of the centimetre, 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 

 o-ooi 1 18 of a gramme per second, may be taken as a current of 

 I ampere. 



(11) That an alternating current of i ampefe 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 de- 

 nominated the volt, being the pressure which, if steadily applied 

 to a conductor whose resistance is I ohm, will produce a current 

 of I 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 

 I '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 I volt shall mean a pres- 

 sure 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 measure- 

 ment 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 Internatioual 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 be- 

 tween 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 



NO. I 140, VOL. 44] 



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 I 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 millimetres 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 volta- 

 meter 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 i& 

 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 immer.<ed 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 novsr 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 o ooiii8. 



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 inter- 

 vals 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. 



NOTES. 

 The International Meteorological Conference at Munich was 

 opened on August 26. Dr. C. Lang, Director of the Bavarian 

 Meteorological Service, was unanimously elected President. 

 Prof. M. W. Harrington (Chief of the United States Weather 

 Bureau) and Prof. E. Mascart (Director of the French Meteoro- 

 logical Service) were elected Vice-Presidents. Mr. R. II, Scott 

 (Secretary of the Meteorological Office), Dr. F. Erk (Munich), 

 and M. L. Teisserenc de Bort (Paris) were elected Secretaries, 

 Thirty members were present, including representatives from 

 Brazil, Queensland, and the United Slates. We hope in a 

 future number to give some account of the proceedings. 



Dr, Barclay, whose death at Simla has been announced, 

 was _working on the Leprosy Commission, and his loss is 



