The figure below shows a multiplate capacitor.
There are three plates A, B, C as shown. From this figure, we can see that the arrangement is equivalent to two parallel capacitors. Hence, the capacity of the capacitor is double. Here we can see that there are only three plates in the capacitor. But the capacitance becomes two times. So, if there are n number of plates in a capacitor, the capacitance will be (n-1) times. In that case, there are (n-1) number of similar capacitors are in parallel.
Suppose A is the area of each plate and d is the distance between two adjacent plates. Hence, as per the formula of capacitance, each unit capacitor will have capacitance.
Here 0 is the absolute permeability of air or vacuum. r is the relative permeability of dielectric medium between two plates.
When there are n number of plates in the multiplate capacitor, the capacitance will be
Multiplate Variable Capacitor
In the multiplate variable capacitor, there are a number of fixed plates. Also, there is the same number of movable plates. One shaft fixes all the movable plates together. Therefore if we rotate the shaft the entire arrangement of fixed plates also rotate. As a result, the active area between fixed and movable plates changes. Therefore, the capacitance of the multiplate variable capacitor also changes. So, we can easily adjust the capacitance as per our requirement. The multiplate variable capacitors are generally air capacitors. Therefore, normally the dielectric medium of this type of capacitor is air.