MEASUREMENT OF RESISTANCES IN ABSOLUTE VALUE. 



by the current itself which passes through the coil, is evidently that 

 which presents the highest degree of simplicity, as it only requires 

 the measurement of a coefficient of induction that is to say, of a 

 length ; and of a velocity of rotation that is to say, of a time. 



The difficulties special to it arise on the one hand from the 

 smallness of the force of induction, and on the other hand from 

 the relative magnitude of the thermoelectric forces produced by the 

 contact of the sliding pieces. These effects are diminished by using 

 contact springs of the same metal as the disc ; and if they are 

 sensibly constant, their effect is eliminated by taking the means 

 of the results obtained for two opposite directions of the field. The 

 smallness of the electromotive force of induction necessitates that 

 the resistance at which the electrodes terminate shall be very small. 

 In order to avoid errors arising from the comparison of two unequal 

 resistances, Lorenz works directly on columns of mercury contained 

 in tubes of 2 cm. to 3 cm. in diameter, carefully calibrated, so as to 

 be able to deduce the value of the mercurial unit by a simple 

 calculation. The resistances used varied from 0^0002 ohms to 

 0*0015 ohms. These mercurial columns should moreover be placed 

 in baths which are kept at a constant temperature. 



Lord Rayleigh and Mrs. Sidgwick* turn the difficulty by a kind 

 of multiplication. Two points A and B of the principal circuit 

 being separated by a resistance R which is greater than that suitable 

 for equilibrium, they are joined by a shunt of considerably greater 

 resistance r. The point on the shunt with the resistance p, starting 

 from A, is then sought which must be interposed between the elec- 

 trodes connected with the spring of the disc so as to produce 

 equilibrium. 



The principal current being I, the branch circuit is equal to 

 T} 



I - , and the difference of potential which corresponds to the 

 R + r 



resistance p is 



The condition of equilibrium is then 



* Lord RAYLEIGH and Mrs. SIDGWICK. Phil. Trans. Land. Roy. Soc. for 

 1883, p. 245. 



