138 REPORT— 1894. 



acting upon the suspended coil in its sighted position is exactly balanced 

 by the force exerted by gravity in Westminster upon the iridio-platinum 

 weight marked A and forming part of the said instrument, 



III. — Standard of Electrical Pressure. 



A standard of electrical pressure denominated one volt, being one 

 hundredth part of the pressure which when applied between the terminals 

 forming part of the instrument marked ' Board of Trade Volt Standard 

 Verified, 1894,' causes that rotation of the suspended portion of the 

 instrument which is exactly measured by the coincidence of the sighting 

 wire with the image of the fiducial mark A before and after application 

 of the pressure and with that of the fiducial mark B during the applica- 

 tion of the pressure, these images being produced by the suspended 

 mirror and observed by means of the eyepiece. 



In the use of the above standards the limits of accuracy attainable 

 are as follows : — 



For the ohm, within one hundredth part of one per cent. 

 For the ampere, within one tenth part of one per cent. 

 For the volt, within one tenth part of one per cent. 



The coils and instruments referred to in this schedule are deposited at 

 the Board of Trade Standardising Laboratory, 8 Richmond Terrace, 

 Whitehall, London. 



Specifications referred to in the foregoing Order in Council. 



SPECIFICATION A. 



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 tlie total electrical quantity which has passed duidng 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 centimetres in diameter, and from 4 to 5 centimetres 

 in depth. 



The anode should be a plate of pure silver some 30 square centimetres 

 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 eSect 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. 



