Membrane potential function as a battery, providing power to operate a variety of "molecular devices" embedded in the membrane. Second, in electrically excitable cells such as neurons and muscle cells, it is used for transmitting signals between different parts of a cell. Signals are generated by opening or closing of ion channels at one point in the membrane, producing a local change in the membrane potential. This change in the electric field can be quickly affected by either adjacent or more distant ion channels in the membrane. Those ion channels can then open or close as a result of the potential change,
Signals, Crane and Subway, Charters Towers railway station mechanical signals were controlled from signal cabins, which dealt with the traffic on a particular block of railway line. Signal cabins coordinated signals that indicated whether or not a section of track was clear of other traffic. Early signals were mechanical, while later signals were operated electrically. The most common form of mechanical signal was the semaphore signal. These consisted of a metal framed tower with one or more arms that could be inclined at different angles, with the arm at the horizontal signalling "danger", or do not proceed. Mechanical signals were usually worked by a wire cable running from