[Illustration: FIG. 131.--Experiment illustrating Lenz’s law. If a copper ring be held in front of an ordinary electromagnet, and the current circulating through the coil of the magnet be in such a direction as to magnetize the core as indicated by the letters S N, then as the current increases in the coil more and more of the lines of force proceeding from N pass through the ring O O from left to right. While the field is thus increasing currents will be induced in the copper ring in the direction indicated by the arrows, such currents tending to set up a field that would pass through the ring from right to left, and would therefore _retard_ the growth of the field due to the electromagnet M.] 4. _A decrease in the number of magnetic lines which pass through a circuit induces a current around the circuit in the positive direction._ The term positive direction is understood to be the direction along which a free N pole would tend to move. 5. _An increase in the number of magnetic lines which pass through a circuit induces a current in the negative direction around the circuit._ The reason for the change of direction of the current for decrease or increase in the number of lines cut, as stated in the fourth and fifth laws, will be seen by aid of the formula given under the second law, viz: electromotive force = (n - n′)/t (1) but by Ohm’s law current = electromotive force / resistance or, I = E/R (2) Substituting (1) in (2) current = ((n - n′)/t)/R or (n - n′)/(Rt) (3) [Illustration: FIG. 132.--Fleming’s rule for direction of induced current. Extend the thumb, forefinger and middle finger of the right hand so that each will be at right angles to the other two. Place the hand in such position that the thumb will point in the direction in which the conductor moves, the forefinger in the direction of the lines of force (N to S), then will the middle finger point in the direction in which the induced current flows.] Now in equation (3) if there be a _decrease_ in the number of lines cut _n′_ will be less than _n_ hence the current will be positive (+); again, if the lines _increase_ _n′_ will be greater than _n_, which will give a minus value, that is, the current will be negative or in a reverse direction. 6. _The approach and recession of a conductor from a magnet pole will yield currents alternating in direction._ Since the strength of the field depends on the proximity to the pole, the approach and recession of a conductor involve an _increase_ and _decrease_ in the rate of cutting of magnetic lines, hence a reversal of current. 7. _The more rapid the motion, the higher will be the induced electromotive force._ In other words, the greater the number of lines cut per unit of time, the higher will be the voltage.