ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 139 



Method of making a Measuremerit. 



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 fi-om 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 cun-ent 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 observed at frequent intervals of time. These observa- 

 tions 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 B. 



On the Preparatiox of the Clark Cell. 



Definition of the Cell. 



The cell consists of zinc or an amalgam of zinc with mercury and of 

 mercury in a neutral saturated solution of zinc sulphate and mercurous 

 sulphate in water, prepared with mercurous sulphate in excess. 



Preparation of the Materials. 



1. The Mercury. — To secure purity it should be first treated with acid 

 in the usual manner, and subsequently distilled in vacuo. 



2. The Zinc. — Take a portion of a rod of pure redistilled zinc, solder 

 to one end a piece of copper wire, clean the whole with glass paper or a 

 steel burnisher, carefully removing any loose pieces of the zinc. Just 

 before making up the cell dip the zinc into dilute sulphuric acid, wash 

 with distilled water, and dry with a clean cloth or filter paper. 



3. The Mercurous Sulphate. — Take mercurous sulphate, purchased as 

 pure, mix with it a small quantity of pure mercury, and wash the whole 



