ACTION OF THE ELECTRO-MAGNET.--The electro-magnet exerts its force in the same manner as a permanent magnet, so far as attraction and repulsion are concerned, and it has a north and a south pole, as in the case with the permanent magnet. An electro-magnet is simply a bar of iron with a coil or coils of wire around it; when a current of electricity flows through the wire, the bar is magnetized. The moment the current is cut off, the bar is demagnetized. The question that now arises is, why an electric current flowing through a wire, under those conditions, magnetizes the bar, or _core_, as it is called. [Illustration: _Fig. 13._ MAGNETIZED FIELD] [Illustration: _Fig. 14._ MAGNETIZED BAR] In Fig. 13 is shown a piece of wire (A). Let us assume that a current of electricity is flowing through this wire in the direction of the darts. What actually takes place is that the electricity extends out beyond the surface of the wire in the form of the closed rings (B). If, now, this wire (A) is wound around an iron core (C, Fig. 14), you will observe that this electric field, as it is called, entirely surrounds the core, or rather, that the core is within the magnetic field or influence of the current flowing through the wire, and the core (C) thereby becomes magnetized, but it is magnetized only when the current passes through the wire coil (A). [Illustration: _Fig. 15._ DIRECTION OF CURRENT] From the foregoing, it will be understood that a wire carrying a current of electricity not only is affected within its body, but that it also has a sphere of influence exteriorly to the body of the wire, at all points; and advantage is taken of this phenomenon in constructing motors, dynamos, electrical measuring devices and almost every kind of electrical mechanism in existence. EXTERIOR MAGNETIC INFLUENCE AROUND A WIRE CARRYING A CURRENT.--Bear in mind that the wire coil (A, Fig. 14) does not come into contact with the core (C). It is insulated from the core, either by air or by rubber or other insulating substance, and a current passing from A to C under those conditions is a current of _induction_. On the other hand, the current flowing through the wire (A) from end to end is called a _conduction_ current. Remember these terms. In this connection there is also another thing which you will do well to bear in mind. In Fig. 15 you will notice a core (C) and an insulated wire coil (B) wound around it. The current, through the wire (B), as shown by the darts (D), moves in one direction, and the induced current in the core (C) travels in the opposite direction, as shown by the darts (D). [Illustration: _Fig. 16._ DIRECTION OF INDUCTION CURRENT] PARALLEL WIRES.--In like manner, if two wires (A, B, Fig. 16) are parallel with each other, and a current of electricity passes along the wire (A) in one direction, the induced current in the wire (B) will move in the opposite direction. These fundamental principles should be thoroughly understood and mastered.