Where is the current density highest when electrodes meet the skin?

The current density is highest at the electrode-skin interface for several reasons related to electrical principles and the behavior of current flow. When an electrical current is applied through electrodes, the area where the electrodes come into contact with the skin presents the highest resistance due to the skin's inherent properties, such as its low electrical conductivity compared to underlying tissues.

At this interface, the concentration of electrical fields is significantly greater as the current tries to pass through the skin barrier. The geometry of the electrodes also plays a role; a smaller electrode area will lead to an increase in current density at the point of contact, as the same amount of current is distributed over a smaller area. This phenomenon can be described by the equation for current density, which shows that current density is inversely proportional to the area through which it flows.

Deeper tissues, the needle insertion point, and the treatment area do not experience the same level of concentration of current. The deeper tissues are beyond the direct influence of the electrodes, and while they receive some current, the density diminishes as it disperses into a larger volume of conductive tissue. The needle insertion point, while it can be an area of higher current focus, does not surpass the concentration found directly at the electrode-skin interface.