NOVEMBEB 27, 1908] 



SCIENCE 



745 



Each, of the tubes must be accurately cali- 

 brated. The correction to be applied to allow 

 for the area of the cross-section of the bore, 

 not being exactly the same at all parts of the 

 tube, must not exceed 5 parts in 10,000. 



The mercury filling the tube must be con- 

 sidered as bounded by plane surfaces placed in 

 contact with the ends of the tube. 



The length of the axis of the tube, the mass 

 of mercury the tube contains, and the elec- 

 trical resistance of the mercury are to be de- 

 termined at a temperature as near to 0° C. as 

 possible. The measurements are to be cor- 

 rected to 0° 0. 



For the purpose of the electrical measure- 

 ments, end vessels carrying connections for 

 the current and potential terminals are to be 

 fitted on to the tube. These end vessels are 

 to be spherical in shape (of a diameter of ap- 

 proximately 4 centimeters), and should have 

 cylindrical pieces attached to make con- 

 nections with the tubes. The outside edge of 

 each end of the tube is to be coincident with 

 the inner surface of the corresponding 

 spherical end vessel. The leads which make 

 contact with the mercury are to be of thin 

 platinum wire fused into glass. The point of 

 entry of the current lead and the end of the 

 tube are to be at opposite ends of a diameter 

 of the bulb ; the potential lead is to be midway 

 between these two points. All the leads must 

 be so thin that no error in the resistance is 

 introduced through conduction of heat to the 

 mercury. The filling of the tube with 

 mercury for the purpose of the resistance 

 measurements must be carried out under the 

 same conditions as the filling for the determi- 

 nation of the mass. 



The resistance which has to be added to 

 the resistance of the tube to allow for the 

 efiect of the end vessels is to be calculated by 

 the formula: 



0.80 



' l,0637r 



a+,i) 



ohm, 



where r^ and r^ are the radii in millimeters of 

 the end sections of the bore of the tube. 



The mean of the calcxilated resistances of at 

 least five tubes shall be taken to determine the 

 value of the unit of resistance. 



For the purpose of the comparison of resist- 

 ances with a mercury tube the measurements 

 shall be made with at least three separate fill- 

 ings of the tube. 



SPECIFICATION n. — SPECIFICATION RELATING TO 

 THE DEPOSITION OF SILVER 



The electrolyte shall consist of a solution of 

 from 15 to 20 parts by weight of silver nitrate 

 in 100 parts of distilled water. The solution 

 must only be used once, and only for so long 

 that not more than 30 per cent, of the silver 

 in the solution is deposited. 



The anode shall be of silver, and the cathode 

 of platinum. The current density at the 

 anode shall not exceed 1/5 ampere per square 

 centimeter and at the cathode 1/50 ampere 

 per square centimeter. 



Not less than 100 cubic centimeters of elec- 

 trolyte shall be used in a voltameter. 



Care must be taken that no particles which 

 may become mechanically detached from the 

 anode shall reach the cathode. 



Before weighing, any traces of solution ad- 

 hering to the cathode must be removed, and 

 the cathode dried. 



SCHEDULE C. — WESTON NORMAL CELL 



The Weston normal cell may be conve- 

 niently employed as a standard of electric 

 pressure for the measurement both of e.m.f. 

 and of current, and when set up in accordance 

 with the following specification, may be taken, 

 provisionally,' as having, at a temperature of 

 20° C, an e.m.f. of 1.0184 volts. 



SPECIFICATION RELATING TO THE WESTON NORMAL 

 CELL 



The Weston normal cell is a voltaic cell 

 which has a saturated aqueous solution of 

 cadmium sulphate (CdSO, 8/3 H,0) as its 

 electrolyte. 



The electrolyte must be neutral to Congo 

 red. 



The positive electrode is mercury. 



The negative electrode of the cell is 

 cadmium amalgam consisting of 12.5 parts by 



' See duties of the scientific committee, sched- 

 ule D. 



