Theory o/'Nobili's Coloured Rings. 21 



where the intensity of the current is greatest, it also is 

 greater than at the margins of the plate. Hence the preci- 

 pitate is relatively more copious at the margins, and conse- 

 quently the radii of the rings are, from this cause, smaller 

 than we should anticipate from calculation. However, since 

 the charges* are not in direct proportion to the intensities of 

 the current, but increase asymptotically from a certain intensity, 

 this influence may be made to disappear in proportion as the 

 intensity of the battery used is increased. For this reason 

 the experiments in the above Tables are arranged according 

 to the number of elements of which the battery used on each 

 occasion consisted. It is at once seen, that, with few ex- 

 ceptions, the greater the number of active elements the more 

 the experiments approximate to calculation. In the first se- 

 ries of experiments, where the unessential resistance is some- 

 what considerable in proportion to the essential, the influence 

 of the charge becomes so evident, that the differences of the 

 values found for m 3 from the mean, evidently follow a definite 

 law. In the subsequent series of experiments this conformity 

 to a law ceases to be perceptible. 



M. du Bois-Reymond has already observed, that the charge 

 upon the plates is less to be ascribed to a deposition of 

 oxygen than to the greater or less thickness of the film of 

 peroxide of lead. It might appear inconsistent with the prin- 

 ciples of the contact-theory, that a greater electromotive ac- 

 tivity should be ascribed to a thicker than to a thinner film. 

 But this is not really the case. When, for instance, a plate 

 of gold is connected, as the anode, with the platinum plate of 

 a Grove's battery, and the deposition of the peroxide of 

 lead commences, this substance will not form a coherent 

 film, but its particles will be arranged in a more diffused 

 manner the less the intensity of the current is at this part of 

 the plate. Hence the electrolyte serves in some parts to con- 

 nect the zinc and platinum circuit (which is composed of the 

 zinc of the Grove's battery and the platinum point forming 

 the cathode) with the gold and platinum circuit, sometimes 

 to connect the former with a circuit of platinum and peroxide 

 of lead. The latter action, which opposes the direction of the 

 battery originally used in the same manner as a charge, will 

 become more evident the nearer the particles of the per- 

 oxide of lead are situated as regards each other, i. e. the 

 thicker the film is which is already deposited. 



For the purpose of investigating which of the two sources 



* By charge (Ladung) the author evidently means the cause of that elec- 

 tromotive reaction to which the term polarization is more frequently ap- 

 plied.— Ed. Phil. Mag. 



