S N 90° 180° 0° 0° 60° 120° 120° 240°300° 360° 1.0 .5 .5 .87 1.0 .87 270° C A B + − + 0 − Damped oscillation caused by resistance in the circuit Battery Square wave + − Rise rate determined by resistor  Ignition voltage 0 E + + − N$_{E}$ C Resistor 11-25 Electronic Oscillation Figure 11-57.  A relaxation oscillator produces a sawtooth wave  output.  Figure 11-56.  The waveform of pulsing DC is a square wave.  Figure 11-55.  Voltage over time of sine waveform electricity created  when a conductor is rotated through a uniform magnetic field, such  as in a generator.  Figure 11-58.  A tank circuit alternately charges opposite plates of  a capacitor through a coil in a closed circuit. The oscillation is an  alternating current that diminishes due to resistance in the circuit.  receiving the energy from the capacitor, the coil stores it by  building up an electromagnetic field. When the capacitor is  fully discharged, the coil stops conducting. The magnetic  field collapses, which induces current flow. The current  charges the opposite plate of the capacitor. When completely  charged, the capacitor discharges into the coil again. The  magnetic field builds again and stops when the capacitor is  fully discharged. The magnetic field collapses again, which  induces current that charges the original plate of the capacitor  and the cycle repeats. This oscillation of charging and discharging the capacitor  through the coil would continue indefinitely if a circuit could  be built with no resistance. This is not possible. However, a  circuit can be built using a transistor that restores losses due  to resistance. There are various ways to accomplish this. The  Hartley oscillator circuit in  Figure 11-59  is one. The circuit  can oscillate indefinitely as long as it is connected to power. When the switch is closed, current begins to flow in the  oscillator circuit. The transistor base is supplied with biasing  current through the voltage divider R$_{A}$ and R$_{B}$. This allows  current to flow through the transistor from the collector to  the emitter, through R$_{E}$ and through the lower portion of the  center tapped coil that is labeled L$_{1}$. The current increasing  through this coil builds a magnetic field that induces current  in the upper half of the coil labeled L$_{2}$. The current from  L$_{2}$ charges capacitor C$_{2}$, which increases the forward bias  of the transistor. This allows an increasing flow of current  through the transistor, R$_{E}$, and L$_{1}$ until the transistor is  saturated and capacitor C$_{1}$ is fully charged. Without force  to add electrons to capacitor C$_{1}$, it discharges and begins  the oscillation in the tank circuit described in the previous  section. As C$_{1}$ becomes fully charged, current to charge C$_{2 }$ reduces and C$_{2}$ also discharges. This adds the energy needed  Oscillation in electronic circuits is accomplished by  combining a transistor and a tank circuit. A tank circuit is  comprised of a capacitor and coil parallel to each other.  [Figure 11-58]  When attached to a power source by closing  switch A, the capacitor charges to a voltage equal to the  battery voltage. It stays charged, even when the circuit to  the battery is open (switch in position B). When the switch  is put in position C, the capacitor and coil are in a closed  circuit. The capacitor discharges through the coil. While 