He then proceeds to recount the experimental evidence of revolving magnets and loops of wire. Following out the old lines of so moving the parts of the system that the magnetic lines were “cut” by the copper conductors, and connecting the latter with a slow-period galvanometer, to test the resultant induction, he found that “the _amount_ of magnetic force” [or _flux_, as we should nowadays call it] “is determinate for the same lines of force, whatever the distance of the point or plane at which their power is exerted is from the magnet.” The convergence or divergence of the lines of force caused, _per se_, no difference in their amount. Obliquity of intersection caused no difference, provided the same lines of force were cut. If a wire was moving in a field of equal intensity, and with a uniform motion, then the current produced was proportional to the velocity of motion. The “quantity of electricity thrown into a current” was, _ceteris paribus_, “directly as the amount of curves intersected.” Within the magnet, running through its substance, existed lines of force of the _same nature_ as those without, exactly equal in _amount_ to those without, and were, indeed, _continuous_ with them. The conclusion must logically be that every line of force is a closed circuit. Having thus established the exact quantitative laws of magneto-electric induction, he then advanced to make use of the induced current as a means of investigating the presence, direction, and amount of magnetic forces--in other words, to explore and measure magnetic fields. He constructed revolving rectangles and rings furnished with a simple commutator, to measure inductively the magnetic forces of the earth. Then he employed the induced current to test the constancy of magnets when placed near to other magnets in ways that might affect their power. Next he considers the fields of magnetic force of two or more associated magnets, and notes how their magnetic lines may coalesce when they are so placed as to constitute parts of a common magnetic circuit. The twenty-ninth series is brought to a close by a discussion of the experimental way of delineating lines of magnetic force by means of iron filings. [Sidenote: THE ELECTROTONIC STATE.]