Root Mean Square Current As mentioned in the previous post , the value of AC is always changing. Then, how do we measure its value??? We could not use the maximum value of the current Io, because it only occurs at a moment. The average current is ZERO . Hence, one effective value that we could use the root mean square value. Irms . The formula for Irms is given by: where Io is maximum current. Irms is the value that is equivalent to a steady direct current . We can investigate this by comparing 2 circuits containing lamp each. circuit 1 uses battery as the source, while circuit 2 uses a.c. supply. AC supply is adjusted so that both lamps are equally bright. At this point, DC and AC supplies are transferring energy to the lamps at the same rate (Power). By equating the Power of both circuits, we can deduce the equation of Irms as above. Therefore, the d.c. voltage equals the r.m.s. value of the a.c. voltage. From the the graph below and the equation of Irms above, we