STANDARDS OF RESISTANCE. 



wires. The capillary part of the tube is immersed in a bath, the 

 temperature of which may be determined by a thermometer T ; a 

 tube A at the side enables us to agitate the liquid by a current of air. 

 We determine, by experiment, either the correction which must be 

 made to bring the resistance to the temperature zero, or the tem- 

 perature at which the resistance is equal to the legal ohm. The 



Fig. 177. 



Fig. 178. 



change of resistance of the mercury in a glass tube, as a function of 

 the temperature, may be calculated by the formula 



i+ 0-0008649 * + 0-000001 1 2/ 2 ). *** 



R = R 



It may, however, be feared that the platinum in the mercury in 

 time partially dissolves, and thus modifies the conductivity. In his 

 recent researches on the practical construction of the legal ohm, 

 M. Benoit adopted a slightly different form (Fig. 178). The ends of 

 the capillary tube terminate in large open tubes. The filling also 

 takes place in a vacuum, but the mercury can be easily renewed. 



Glass instruments being fragile, the practical standards are usually 

 constructed with metal wires ; alloys are to be preferred to pure 



