132 REPORT — 1893. 



anode from falling on to the kathode the anode should be wrapped 

 roand 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 fifteen parts by weight of the nitrate to eighty- five parts 

 of water. 



The resistance of the voltameter changes somewhat as the current 

 passes. To prevent these changes having too great an eiFect 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 mahing 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 -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. 



Specification eefeeked to in Resolution 14. 

 Definition of the Cell. 



The cell consists of zinc and mercury in a saturated solution of zinc 

 sulphate and mercurous sulphate in water, prepared with mercurous 

 sulphate in excess, and is conveniently contained in a cylindrical glass 

 vessel. 



