[Scuderia_Turini]

@CMOTD - OK... Alternating Current is by definition not a fixed voltage, the voltage varies between (say)  +15 volts through to zero volts and to -15 volts and so on.  A Direct Current voltage remains at a fixed voltage (although that can be varied!). The DC equivalent of an AC voltage is known as the Root Mean Square voltage (RMS).  RMS may ring a bell as it is used in rating the power output of sound systems (Watts RMS). 

Its all about working out the average/mean voltage of a sine wave to end up with a voltage that will result from rectiifiying the AC. 

Rectification is the technique of channelling alternating current into uni-directional current so as to produce direct current. 

Pictures tell a thousand words so here is a sine-wave diagram from Wikipedia page - Wiki RMS
You can see the AC voltage swing from positive to negative, with the RMS indicated.

Hope this helps,  Leo