174 Mr. Herschel on certain Motions produced in Fluid [Sept. 



where it is exerted. The magnetic effect is proportional 

 (cceteris paribus) to the absolute quantity of electricity in motion 

 present at once, (or at any indivisible instant of time) in a 

 given portion of the conducting wire, or within the sphere of 

 action of the needle, that is, to its density.* To establish or 

 refute this distinction, will require experiments which it is easy 

 to imagine, but which I have not yet had an opportunity of 

 making. At first sight, indeed, the phenomena in question 

 present a considerable analogy to the electro-magnetic vortices 

 observed in the fluid metals ; but on presenting very powerful 

 magnets to the mercury, while under the circumstances above 

 described, in various positions, I have never- been able to per- 

 ceive any influence exerted by them in accelerating, retarding, 

 or deviating the currents ; and moreover, these are incompara- 

 bly more forcible in proportion to the electric powers used, than 

 the motions produced by the action of magnets. 



14. In consequence of this superior energy of action, the 

 phsenomena which form the subject of this Paper, furnish a 

 test, perhaps, the most sensible yet known of the develope- 

 ment of feeble Voltaic powers. I constructed a small battery 

 of zinc and copper wires twisted together, each pair being two 

 inches long from the point of junction, and the wires -^ of an 

 inch thick. Ten pairs of these, excited by extremely dilute 

 nitric acid, caused a rapid rotation in mercury, interposed under 

 sulphuric acid between the poles, and a regular advance of 

 globules of that metal towards the negative pole. The rotation 

 continued with considerable force, when the wires were so far 

 withdrawn as to have only their extremities in contact with the 

 liquid in the cells, in which case the surface exposed by each 

 pair to the action of the acid could not exceed -J§ of a square 

 inch. Nay, so delicate is this indication, that the electricity 

 developed by bringing the extremities of a thin zinc and copper 

 wire in contact with a glass merely moistened with the above 

 mentioned dilute acid, is abundantly sufficient to cause an 

 immediate and unequivocal rotation in an ounce or two of mer- 

 cury properly exposed to its action. By this means, indeed, the 

 feeblest electrical excitement may be placed in evidence. I have 



* In these expressions I have conceived electricity as being transmitted through 

 conductors according to the laws of a gas of high, but variable elasticity through pipes 

 more or less obstructed, a supposition which will represent many of the phsenomena. 

 The sluggish electricity of a single pair of plates may be compared to air, rendered 

 dense and less elastic by excessive cold, while the active charge of a powerful battery, or 

 the spark of an ordinary electrical machine, is in this view assimilated to air with all its 

 energies exalted, and its density diminished by violent heat. The same quantity in 

 weight may pass through the same conducting pipe in the same time ; but in the one 

 case the motion of each molecule will be comparatively much slower, and the actual 

 quantity present at any instant of the discharge (c g. an inch in length) of the conduc- 

 tor, much greater than in the other. I am well aware that this is merely an analogical 

 representation of facts, and of course inaccurate, but it serves to explain the distinction 

 in the text. 



