In 1851, from July to December, Faraday was actively at work in the laboratory. The results constitute the material for the twenty-eighth and twenty-ninth (the last) series of the “Experimental Researches.” In these he returned to the subject with which the first series had opened in 1831: the induction of electric currents by the relative motion of magnets and conducting wires. These two memoirs, together with his Royal Institution lecture of January, 1852, “On the Lines of Magnetic Force,” and the paper “On the Physical Character of the Lines of Magnetic Force” (which he sent to the _Philosophical Magazine_, as containing “so much of a speculative and hypothetical nature”), should be read, and re-read, and read again, by every student of physics. They are reprinted at the end of the third volume of the “Experimental Researches.” In the opening of the twenty-eighth memoir he says:-- From my earliest experiments on the relation of electricity and magnetism, I have had to think and speak of lines of magnetic force as representations of the magnetic power--not merely in the points of quality and direction, but also in quantity.... The direction of these lines about and amongst magnets and electric currents is easily represented and understood in a general manner by the ordinary use of iron filings. A point equally important to the definition of these lines is, that they represent a determinate and unchanging amount of force. Though, therefore, their forms, as they exist between two or more centres or sources of power, may vary very greatly, and also the space through which they may be traced, yet the sum of power contained in any one section of a given portion of the lines is exactly equal to the sum of power in any other section[54] of the same lines, however altered in form or however convergent or divergent they may be at the second place.... Now, it appears to me that these lines may be employed with great advantage to represent the nature, condition, and comparative amount of the magnetic forces, and that in many cases they have, to the physical reasoner, at least, a superiority over that method which represents the forces as concentrated in centres of action, such as the poles of magnets or needles; or some other methods, as, for instance, that which considers north or south magnetisms as fluids diffused over the end, or amongst the particles, of a bar. No doubt any of these methods which does not assume too much will, with a faithful application, give true results. And so they all ought to give the same results, as far as they can respectively be applied. But some may, by their very nature, be applicable to a far greater extent, and give far more varied results, than others. For, just as either geometry or analysis may be employed to solve correctly a particular problem, though