The resistance of a metallic conductor is directly proportional  to its length. The longer the length of a given size of wire, the  greater the resistance.  Figure 9-5  shows two wire conductors  of different lengths. If 1 volt of electrical pressure is applied  across the two ends of the conductor that is 1 foot in length  and the resistance to the movement of free electrons is  assumed to be 1 ohm, the current flow is limited to 1 ampere.  If the same size conductor is doubled in length, the same  electrons set in motion by the 1 volt applied now find twice  the resistance. Electrical energy can be produced through a number of  methods. Common methods include the use of light, pressure,  heat, chemical, and electromagnetic induction. Of these  processes, electromagnetic induction is most responsible for  the generation of the majority of the electrical power used  by humans. Virtually all mechanical devices (generators and  alternators) that produce electrical power employ the process  of electromagnetic induction. The use of light, pressure,  heat, and chemical sources for electrical power is found on  aircraft but produce a minimal amount of all the electrical  power consumed during a typical flight. produces electricity using two different metals in a chemical  solution like alkaline electrolyte. A chemical reaction exists  between the metals which frees more electrons in one metal  than in the other. Heat used to produce electricity creates the thermoelectric  effect. When a device called a thermocouple is subjected to  heat, a voltage is produced. A thermocouple is a junction  between two different metals that produces a voltage related  to a temperature difference. If the thermocouple is connected  to a complete circuit, a current also flows. Thermocouples are  often found on aircraft as part of a temperature monitoring  system, such as a cylinder head temperature gauge.  Electromagnetic induction is the process of producing a  voltage (EMF) by moving a magnetic field in relationship  to a conductor. As shown in  Figure 9-6,  when a conductor  (wire) is moved through a magnetic field, an EMF is produced  in the conductor. If a complete circuit is connected to the  conductor, the voltage also produces a current flow.  In brief, light can produce electricity using a solar cell  (photovoltaic cell). These cells contain a certain chemical  that converts light energy into voltage/current.  Using pressure to generate electrical power is commonly  known as the piezoelectric effect. The piezoelectric effect  (piezo or piez taken from Greek: to press; pressure; to  squeeze) is a result of the application of mechanical pressure  on a dielectric or nonconducting crystal. Chemical energy can be converted into electricity, most  commonly in the form of a battery. A primary battery  One single conductor does not produce significant voltage/ current via electromagnetic induction.  [Figure 9-6]  In  practice, instead of a single wire, a coil of wire is moved  through the magnetic field of a strong magnet. This produces  a greater electrical output. In many cases, the magnetic field  is created by using a powerful electromagnet. This allows  for the production of a greater voltage/current due to the  stronger magnetic field produced by the electromagnet when  compared to an ordinary magnet. Please note that this text often refers to voltage/current in  regards to electrical power. Remember voltage (electrical  pressure) must be present to produce a current (electron flow).  Hence, the output energy generated through the process  of electromagnetic induction always consists of voltage.  Electromagnetic Generation of Power + + 2 feet 0.5 Amp ( 2 Ohms) 1 foot 1 Amp (1 Ohm) N S Motion of conductor EMF Figure 9-6.  Inducing an EMF in a conductor. Figure 9-5.  Resistance varies with length of conductor. 9-5