Adding a "top hat" to a tower modifies the current distribution on the tower by terminating the top of the tower in a capacity, as opposed to leaving the top of the tower open. This causes the current at the top of the tower to increase from a near zero value to a value determined by the amount of terminating capacity. From a mathematics viewpoint, the ideal top hat would be a horizontal disc because the capacity of a disc can be easily calculated. In theory, the size of the top hat can be estimated by considering the tower to be an open transmission line, and calculating the effective capacity of the missing phantom section. In addition to using a tower skirt to electrically shorten a tower for the purpose of detuning, the skirt is sometimes used to alter the current distribution on the tower and thus modify the vertical radiation pattern.