52 REPORTS ON THE STATE OF SCIENCE, 



I'he International Ohm. 



The international ohm shall be equal to the resistance offered to an 

 unvarying electric current by a column of mercury at the temperature of 

 meltin" ice, 14'4521 grammes in mass, of a constant cross sectional area, 

 and of 106-300 centimetres in length, arranged in accordance with the 

 following specification. 



The column of mercury shall be of circular section, or nearly so, and 

 shall be contained in a tube of suitable glass which has been carefully 

 annealed. The tube shall be straight to the eye, and the maximum 

 variation in its area of cross section shall not exceed 2 parts in 100. The 

 tube is to be carefully calibrated, and the correction for its conicality 

 determined. 



In determining the weight of mercury contained by the tube when 

 filled at the temperature of melting ice, the column of mercury is to be 

 bounded by planes at the terminal cross sections of the tube. The tube 

 should not be unduly heated, and it should be filled with mercury by 

 exhaustion of air. 



The axial length of the tube should be measured at 0° C. if possible, 

 otherwise the coefficient of expansion of the glass should be determined 

 and the axial length of the tube at 0° C. calculated from axial measure- 

 ments made very near to that temperature. To facilitate measurements 

 of the axial length, the ends of the tube should be ground very slightly 

 convex. 



For the electrical measurements the ends of the tube are to be 

 connected to spherical bulbs of glass, the ends of the tube forming, 

 approximately, portions of the internal spherical surfaces of the bulbs. 

 Each bulb is to be provided with current and potential leads, the point 

 of entry of the former being at the opposite end of a diameter of the bulb 

 from an end of the tube. The potential lead shall be situated in a 

 plane midway between the point of entry of the current lead and the 

 end of the tube, and at right angles to the line connecting them. 



Contact with the mercury shall be made by means of platinum 

 wires. 



The diameter of a bulb is to be from 30 to 33 times the diameter 

 of that end of the tube to which it is connected. 



If L is the axial length in centimetres of the mercury column con- 

 tained by the tube at 0° C., W the weight of the column in grammes, and 

 c the correction for the conicality of the tube, the resistance of the 

 column at 0° C. is 



" (IWO-OP • ^^ = 0-001278982 c ^ international ohms. 



When the spherical bulbs are fitted to the ends of the tube and the 

 whole filled with mercury, if r is the mean radius of the tube and r,, rj, 

 the mean radii in centimetres of the terminal sections, the resistance a,t 

 0° C. between the potential leads is 



L2 



0-001278982 ^ 



( rx + r., \ I * 



O-SOr^ \ r, rg / }• international ohms, 



