include copper, silver, aluminum, and gold. The term “free  electron” describes a condition in some atoms where the  outer electrons are loosely bound to their parent atom. These  loosely bound electrons are easily motivated to move in a  given direction when an external source, such as a battery,  is applied to the circuit. These electrons are attracted to the  positive terminal of the battery, while the negative terminal  is the source of the electrons. So, the measure of current is  actually the number of electrons moving through a conductor  in a given amount of time. The internationally accepted unit for current is the ampere  (A). One ampere (A) of current is equivalent to 1 coulomb  (C) of charge passing through a conductor in 1 second. One  coulomb of charge equals 6.28  ×  10$^{18}$ electrons. Obviously,  the unit of amperes is a much more convenient term to use  than coulombs. The unit of coulombs is simply too small to  be practical.  When current flow is in one direction, it is called direct  current (DC). Later in the text, the form of current that  periodically oscillates back and forth within the circuit is  discussed. The present discussion is concerned only with  the use of DC. It should be noted that as with the movement  of any mass, electron movement (current flow) only occurs  when there is a force present to push the electrons. This force  is commonly called voltage (described in more detail in the  next section). When a voltage is applied across the conductor,  an electromotive force creates an electric field within the  conductor, and a current is established. The electrons do not  move in a straight direction, but undergo repeated collisions  with other nearby atoms within a conductor. These collisions  usually knock other free electrons from their atoms, and these  electrons move on toward the positive end of the conductor  with an average velocity called the drift velocity, which is  relatively low speed. To understand the nearly instantaneous  speed of the effect of the current, it is helpful to visualize a  long tube filled with steel balls.  [Figure 9-3]  It can be seen that a ball introduced in one end of the tube,  which represents the conductor, immediately causes a ball  to be emitted at the opposite end of the tube. Thus, electric  current can be viewed as instantaneous, even though it is the  result of a relatively slow drift of electrons.  There are two competing schools of thought regarding the  flow of electricity. The two explanations are the conventional  current theory and the electron theory. Both theories  describe the movement of electrons through a conductor.  They simply explain the direction current moves. Typically  during troubleshooting or the connection of electrical circuits,  the use of either theory can be applied as long as it is used  consistently. The Federal Aviation Administration (FAA)  officially defines current flow using electron theory (negative  to positive).  The conventional current theory was initially advanced by  Benjamin Franklin, who reasoned that current flowed out of  a positive source into a negative source or an area that lacked  an abundance of charge. The notation assigned to the electric  charges was positive (+) for the abundance of charge and  negative (−) for a lack of charge. It then seemed natural to  visualize the flow of current as being from the positive (+)  to the negative (−). Later discoveries were made that proved  that just the opposite is true. Electron theory describes what  actually happens in the case of an abundance of electrons  flowing out of the negative (−) source to an area that lacks  Conventional Current Theory and Electron Theory  E I X R E I X R E I X R A B C To find I (amperes),  place thumb over I  and divide E by R  as indicated. To find R (ohms),  place thumb over  R and divide as  indicated. To find E (volts),  place thumb over  E and multiply as  indicated. Figure 9-2.  Ohm's Law chart.  Figure 9-3.  Electron flow. 9-3