_=443. Direct and Alternating Currents.=_ A current that flows steadily in one direction is said to be a _direct_ current. A cell gives a direct current when the circuit is closed. When the current passes in one direction for an instant, and then reverses immediately and flows in the opposite direction, it is said to _alternate_. The induced current which flowed through the galvanoscope in Exp. 182 was an alternating one. Currents of this class have great practical uses. _=444. Self-Induction; Extra Currents.=_ It has been shown that a magnetized coil can act through space and induce a current in a neighboring coil. The lines of force which reach out from an electromagnet will generate a current in any conductor which happens to be in the field, or which is moved across the lines. It is evident, then, since the lines of force from each turn of a coil cut all the other turns of the same coil, that each turn acts as a conductor placed in the field of every other turn. The instant a current begins to flow through a coil, there is an inverse current of self-induction started in the coil, which opposes the current in the cell. When the circuit is broken, this _extra current_, as it is also called, is a direct one and adds its strength to that of the current from the cell; as this takes place at the instant the circuit is broken, a bright spark is seen at the key, and this shows that the E. M. F. of this extra current is high. Practical uses are made of it. CHAPTER XXVI. THE PRODUCTION OF MOTION BY CURRENTS. _=445. Currents and Motion.=_ We have seen, in the experiments on induced currents, that a current of electricity can be generated by properly moving magnets near coils of wire. (See Dynamo-electric Machines.) Can we reverse this process? Can motion be produced by the electric current? =EXPERIMENTS 183-190. To study the production of motion by means of the electric current.= _Apparatus._ The support, including base, rod, and support wire, S W (Fig. 144.) Coils of wire (No. 89, 90); iron cores for coils; cell; key; connecting wires; compass; current reverser; bar magnet; horseshoe magnet. [Illustration: Fig. 144.] =EXPERIMENT 183. Motion produced with a hollow coil and a piece of iron.= =446. Directions.= (A) Arrange as in Fig. 144. Coil H (No. 90) is to be used as a pendulum, and can be supported by fastening a string to it, the upper end of which should be tied to S W. Connect the ends of H with K and D C. There will be a slight magnetic field about H as soon as the circuit is closed. (B) Hold I C near the end of the coil. Close the circuit for an instant. Is there any motion produced in H? While the motion will be slight, there should be enough to be noticed if the cell is strong.