Most AC meters display the effective value of the AC. In  almost all cases, the voltage and current ratings of a system  or component are given in effective values. In other words,  the industry ratings are based on effective values. Peak and  instantaneous values, used only in very limited situations,  would be stated as such. In the study of AC, any values given  for current or voltage are assumed to be effective values  unless otherwise specified. In practice, only the effective  values of voltage and current are used. The effective value is equal to .707 times the peak (maximum)  value. Conversely, the peak value is 1.41 times the effective  value. Thus, the 110 volt value given for AC is only 0.707  of the peak voltage of this supply. The maximum voltage is  approximately 155 volts (110 × 1.41 = 155 volts maximum). How often the AC waveform repeats is known as the AC  frequency. The frequency is typically measured in cycles per  second (CPS) or hertz (Hz). One Hz equals one CPS. The  time it takes for the sine wave to complete one cycle is known  as period (P). Period is a value or time period and typically  measured in seconds, milliseconds, or microseconds. It  should be noted that the time period of a cycle can change  from one system to another; it is always said that the cycle  completes in 360° (related to the 360° of rotation of an AC  alternator).  [Figure 9-13] increases to a maximum negative value, and again decreases  to zero. The cycle repeats until the voltage is no longer  available. There are two alternations in a complete cycle:  the positive alternation and the negative. It should be noted  that the polarity of the voltage reverses for each half cycle.  Therefore, during the positive half cycle, the electron flow  is considered to be in one direction; during the negative half  cycle, the electrons reverse direction and flow the opposite  way through the circuit.  A cycle is a completion of a pattern. Whenever a voltage  or current passes through a series of changes, returns to the  starting point, and then repeats the same series of changes,  the series is called a cycle. When the voltage values are  graphed, as in  Figure 9-14,  the complete AC cycle is  displayed. One complete cycle is often referred to as the sine  wave and said to be 360°. It is typical to start the sine wave  where the voltage is zero. The voltage then increases to a  maximum positive value, decreases to a value of zero, then The frequency is the number of cycles of AC per second  (CPS). The standard unit of frequency measurement is the  Hz.  [Figure 9-15]  In a generator, the voltage and current  pass through a complete cycle of values each time a coil  or conductor passes under a north and south pole of the  magnet. The number of cycles for each revolution of the  coil or conductor is equal to the number of pairs of poles. Frequency Defined Cycle Defined Figure 9-14.  Cycle of voltage.  One cycle one period (time) One wavelength (distance) Second cycle Vertical scale (voltage) Horizontal scale (time) Positive alternation 1T 2T 3T 4T Negative alternation 360° 1 second frequency = 2 cycles per second 1T 0° 90° 180° 270° 360° 1 second frequency = 8 cycles per second 270° 180° 0° 90° Figure 9-15.  Frequency in cycles per second.  Figure 9-13.  Values of AC. 9-10 Average value − Average = 0.637 peak RMS (effective) = 0.707 peak Peak to peak = 2 peaks RMS value Peak value Peak-to-peak value 0