302 MEASUREMENT OF CURRENTS. 



In strictness it would be necessary to take a tube of infinite 

 length, but we should first calculate by formula (12) of (366) the 

 correction for the potential at the two ends of the tube. This 

 correction could be determined experimentally by the rotation ob- 

 served with the tubes at each end of the tube actually employed. 

 The maximum action is, in fact, produced on the liquid layers 

 nearest the currents. If the least distance from each end of the 

 tube to a winding is greater than ten times the radius of the winding, 

 the solid angles, which correspond to the values of V and V, are less 



4?"" 



than , and the corresponding correction does not amount to 



400 



0*005. 



If the current is reversed, double the rotation of the liquid is 

 observed. 



Gordon,* Becquerel,f and Lord RayleighJ have determined the 

 constant of bisulphide of carbon, but their measurements cannot be 

 directly compared * for the first refers to the green ray of thallium, 

 the two others to the ray D of the spectrum ; and the experiments 

 were made at different temperatures. According to Bichat, the 

 rotation diminishes by about 0*0013 for a rise of temperature of i. 

 If we correct for temperature, and reduce the results to the same 

 wave length by Verdet's || formula, we find for unit difference of 

 potential (COS) that the rotation of bisulphide of carbon will be 



At zero. At 18. 



0*04260 0*04163 Gordon. 

 0*04626 0*04520 H. Becquerel. 

 0*04298 0*04200 Lord Rayleigh. 



A coil of 5000 turns, with a current of an ampere, will give a 

 rotation of about 275', or a double rotation of 550' that is to say, 

 an approximation of 0*002 if the error of reading is i'. 



This optical method has great advantages. It always gives the 

 actual value of the intensity, and there is no perturbation -due to 



* GORDON. . Phil. Trans, for 1877, p. I. 



+ H. BECQUEREL. Ann. de Chim. et de Phys. [5], Vol. xxvu., p. 312. 1882. 

 J Lord RAYLEIGH. Proceedings of the Roy. Soc., No. 232. 1884. 

 BICHAT. Journal de Physique, Vol. vin., p. 204. 1879. 

 || VERDET. Comptes rendus de PAcad. de Science, Vol. LVIL, p. 670. 1863. 

 Ann. de Chim. et de Phys. [3], Vol. LXIX., p. 415. 



