228 Intelligence and Miscellaneous Articles. 



Rive, Lenz, Poggendorff, and Vorsselmann de Heer* ; and it has been 

 proved that the polarization was not completely destroyed by this 

 means. But it may be hoped that it can be completely annulled by 

 diminishing the duration of the alternate currents, which diminishes 

 the perturbing electrolytic effect — and by increasing the size of the 

 electrodes, which diminishes the intensity of the currents, and con- 

 sequently the polarization, which is nearly proportional to it. It 

 will be acknowledged that this object is attained when the resistance 

 of the liquid follows Ohm's law — that is to say, when this resist- 

 ance could be replaced by that of the wire of a metallic rheostat, 

 whatever the intensity. 



In order to apply this method, three apparatus are indispensable — 

 an induction-apparatus which gives currents alternately in opposite 

 directions, an apparatus for their measurement which admits of these 

 currents being compared, and also a rheostat. 



For the induction-apparatus the siren by Weber and R. Kohl- 

 rausch was chosen f. It is an ordinary siren having the moveable 

 disk of magnetized steel. This magnet rotates inside a rectangular 

 multiplier similar to those of ordinary galvanometers; and its rotation 

 produces in the wire of the multiplier induced currents which change 

 their direction at each half-revolution. 



There is no better instrument with which to measure these alter- 

 nate currents than Weber's bifilar dynamometer. Its deviation, Z, 

 is proportional to the square of the intensity of the induced currents 

 which traverse it ; and these latter vary with the velocity of the rota- 

 tion of the disk of the siren, a velocity which may be measured 

 from the sound produced. When, for all velocities of the siren, 

 the resistance of the column of liquid could be replaced by that of 

 the same length of wire of the rheostat, it was clear that the polari- 

 zation had disappeared, and that the rheostat measured the special 

 resistance of the liquid. Now this is what always occurred when 

 the electrodes had the greatest dimensions (2900 square millims.). 



In these experiments the velocity of the disk varied from 2*3 to 

 76*9 turns per second; the electromotive force of the induced cur- 

 rent varied consequently from -fa to J of that of a Grove's element. 

 The authors wished to operate on weaker currents. They used the 

 thermoelectric currents produced by an iron-copper element, which 

 gives for 1° of difference of temperature an electromotive force equal 

 to l a 5 * of that of Grove, this difference not having in their expe- 

 riments exceeded 4°. This current passed through a solution of 

 sulphate of zinc 83 millims. in length and having a section of 2400 

 square millimetres ; in order to measure it the dynamometer had to be 

 replaced by a very sensitive galvanometer. In these experiments the 

 electromotive force descends to ^2 9W0 °^ Grove's, and the liquid 

 follows Ohm's laws in all cases. 



The memoir ends with a determination of the resistance of water 

 acidulated by sulphuric acid at different degrees of concentration. 

 By collecting all the experiments at the same temperature, the fol- 



* Pogg. Ann. vols, xlv., xlviii., lii., liii. 



t Memoires de la Socicte Roy ale de Saxe. vol. vi. p. 699. 



