ox TRACitlCAL STANDARDS t^OR ELECTRICAL MEAPUREMEiXTS. 53 

 correct to 1 per cent, of the added resistance 



0-001278982 ~[ 0-8 Or (-~^^')Y 



* The end correction factor is given in these formulst as 080 : this Vtiiuo is. 

 however, subject to amendment. 



The electrical measuronients are to be carried out at 0° C, the tube and 

 spherical vessels being surrounded by melting ice and about 1 5 centi- 

 metres below the upper surface of the ice. The connecting wires employed 

 for the current and potential leads must be thin, the flow of heat through 

 them to the mercury being insufficient to warm the mercury so as to 

 produce appreciable error. 



The insulation resistance between the mercury column and the ice 

 surrounding the tube must not be less than 10,000,000 ohms. 



The current employed in comparing the mercury resistance with other 

 resistances shall be limited by the condition that the mercury shall not 

 be warmed sufficiently to produce appreciable error. 



The mean of at least five tubes must be taken to determine the value 

 of the mercury unit. 



The mean of at least three fillings shall be taken as the value of the 

 resistance of a tube. 



Specification /or the Practical A-ppUcation of the Definition of the 

 International Ampere, 



Conditions under which silver is to be deposited to measure currents 

 from 05 to 8 amperes: — 



The solution shall consist of from 15 parts to 25 parts by weight of 

 pure crystallised ailver nitrate in 100 parts of distilled water free from 

 chlorine. It shall be used for one determination of current only. 



In cases in which it is desired to measure a current of about 1 ampere 

 the anode shall consist of a disc or plate of pure silver about 60 square 

 centimetres in area and 3 or 4 millimetres in thickness. It is supported 

 by a silver rod riveted through its centre. The anode shall be inserted 

 into a cup of filter paper separately supported. 



The kathode shall consist of a platinum bowl about 10 centimetres in 

 diameter and 7 centimetres in depth. 



About 300 cubic centimetres of the silver-nitrate solution are to be 

 placed in the kathode bowl, and the anode is to be supported near the top 

 of the solution and is to be just covered by it. Not more than from 7 to 

 10 grammes of silver should be deposited. 



(For the measurement of smaller currents, say from j to g^ ampere, a 

 bowl holding about 60 cubic centimetres of solution may be used, the 

 anode being proportionately reduced in size and from 2 to 3 grammes of 

 silver being deposited.) 



The deposit should be rinsed with distilled water free from chlorine 

 until the addition of a drop of neutral solution of sodium chloride in water, 

 to the wash water, produces no milkiness. The kathode bowl is then 

 nearly filled with distilled water and left for at least three hours ; it should 

 be rinsed three times, the last of these wash waters remaining in the bowl 

 for ten minutes. This last wash water should give no milkiness when 

 ftdded to a neutral solution of sodium chloride in water. The deposit is 



