_=438. Discussion of Exps. 180, 181.=_ When a current suddenly begins to flow through a coil, the effect upon a neighboring coil is the same as that produced by suddenly bringing a magnet near it; and when the current stops, the opposite effect is produced. We may consider that when the inducing circuit is closed, the lines of force shoot out through the turns of the outside coil. Upon opening the circuit the lines of force cease to exist; that is, we may imagine them drawn in again. [Illustration: Fig. 142.] _=439. Direction of Induced Current.=_ Fig. 142 shows the magnet on its way into the coil; the number of lines of force is increasing in the coil, and the induced current passes in an anti-clockwise direction when looking down into the coil along the lines of force. This produces an _indirect_ current. If a current from a cell were passed through the coil in the direction of this indirect current, the lower end of a bar of iron would become a S pole. (See § 428.) _=440. Laws of Induction.=_ (1) An increase in the number of lines of force that pass through a closed circuit produces an indirect induced current; while a decrease produces a direct one. (See § 428.) (2) The E. M. F. of the induced current is equal to the rate of increase or decrease in the number of lines of force that pass through the circuit. (3) A constant current produces no induced current, provided there is no motion. (4) Closing a circuit produces an indirect current. (5) Opening a circuit produces a direct current. (6) _Lenz's Law._ Induced currents have a direction that tends to stop the motion that produces them. _=441. Primary and Secondary Currents.=_ In the preceding experiments in induction, it must be kept in mind that the current from the cell did not pass through the galvanoscope. There were two entirely separate circuits, in no way connected. The _primary_ current comes from the cell, while the _secondary_ current is an induced one. [Illustration: Fig. 143.] =EXPERIMENT 182. To see what is meant by alternating currents.= =442. Directions.= (A) Arrange as in Fig. 143. Connect coil H with A G, as before. Place one pole of H M against the end of the core I C, hold H with one hand, and with the other quickly push the other pole of H M onto the core. This should produce a momentary current through A G, first in one direction, and then in the other. Let the needle come to rest. (B) Move H M back and forth upon the end of I C, changing its polarity rapidly. A minute's practice will enable you to slide the core from one pole of H M to the other and back again rapidly--3 complete vibrations per second being about right. The needle should be parallel to the coil of A G, and if properly done, the needle will be made to vibrate back and forth slightly at each change in the polarity of I C.