electrons or the positive (+) source. Both conventional flow  and electron flow are used in industry.  Voltage is most easily described as electrical pressure force.  It is the electromotive force (EMF), or the push or pressure  from one end of the conductor to the other, that ultimately  moves the electrons. The symbol for EMF is the capital letter  E. EMF is always measured between two points and voltage  is considered a value between two points. For example,  across the terminals of the typical aircraft battery, voltage  can be measured as the potential difference of 12 volts or  24 volts. That is to say that between the two terminal posts  of the battery, there is a voltage available to push current  through a circuit. Free electrons in the negative terminal of  the battery move toward the excessive number of positive  charges in the positive terminal. The net result is a flow or  current through a conductor. There cannot be a flow in a  conductor unless there is an applied voltage from a battery,  generator, or ground power unit. The potential difference,  or the voltage across any two points in an electrical system,  can be determined by:  The voltage at one point is 14 volts. The voltage at a second  point in the circuit is 12.1 volts. To calculate the voltage drop,  use the formula above to get a total voltage drop of 1.9 volts. Figure 9-4  illustrates the flow of electrons of electric current.  Two interconnected water tanks demonstrate that when a  difference of pressure exists between the two tanks, water  flows until the two tanks are equalized.  Figure 9-4  shows  the level of water in tank A to be at a higher level, reading  10 pounds per square inch (psi) (higher potential energy),  than the water level in tank B, reading 2 psi (lower potential  energy). Between the two tanks, there is 8 psi potential  difference. If the valve in the interconnecting line between the  tanks is opened, water flows from tank A into tank B until the  level of water (potential energy) of both tanks is equalized.  It is important to note that it was not the pressure in tank A The best conductors are materials, chiefly metals, that possess  a large number of free electrons. Conversely, insulators are  materials having few free electrons. The best conductors are  silver, copper, gold, and aluminum, but some nonmetals, such  as carbon and water, can be used as conductors. Materials  such as rubber, glass, ceramics, and plastics are such poor  conductors that they are usually used as insulators. The  current flow in some of these materials is so low that it is  usually considered zero. There is no distinct dividing line between conductors and  insulators; under the proper conditions, all types of material  conduct some current. Materials offering a resistance to  current flow midway between the best conductors and the  poorest conductors (insulators) are sometimes referred to  as semiconductors and find their greatest application in the  field of transistors. The two fundamental properties of current and voltage  are related by a third property known as resistance. In any  electrical circuit, when voltage is applied to it, a current  results. The resistance of the conductor determines the  amount of current that flows under the given voltage. In  general, the greater the circuit resistance, the less the current.  If the resistance is reduced, then the current will increase.  This relation is linear in nature and is known as Ohm’s Law.  An example would be if the resistance of a circuit is doubled,  and the voltage is held constant, then the current through the  resistor is cut in half.  that caused the water to flow; rather, it was the difference  in pressure between tank A and tank B that caused the flow.  This comparison illustrates the principle that electrons move,  when a path is available, from a point of excess electrons  (higher potential energy) to a point deficient in electrons  (lower potential energy). The force that causes this movement  is the potential difference in electrical energy between the two  points. This force is called the electrical pressure (voltage),  the potential difference, or the electromotive force (electron  moving force). The resistance of a metallic conductor is dependent on the  type of conductor material. It has been pointed out that certain  metals are commonly used as conductors because of the  large number of free electrons in their outer orbits. Copper  is usually considered the best available conductor material,  since a copper wire of a particular diameter offers a lower  resistance to current flow than an aluminum wire of the same  diameter. However, aluminum is much lighter than copper,  and for this reason, as well as cost considerations, aluminum  is often used when the weight factor is important. Factors Affecting Resistance Example Resistance Electromotive Force (Voltage) V$_{1}$ – V$_{2}$ = V$_{Drop}$ Figure 9-4.  Difference of pressure.  A B 9-4