Method and apparatus for detecting and obtaining information about (particularly locating) an event (E), i.e. a change in a variable, for example a liquid leak. In a preferred embodiment, two conductors (11,12) follow an elongate path and are connected to each other through a power source, preferably a constant current source (15). In the absence of an event, the two conductors (11, 12) are electrically insulated from each other at all points along the path. When an event (E) occurs, the two conductors (11, 12) become electrically connected at a connection point at which the event occurs, thus completing a test circuit in which a current of known size then flows. When using a constant current source (15), the size of the current does not depend on the resistance of the connection. One of the conductors is a locating member having known impedance characteristics along its length, and by measuring the voltage drop between the connection point and one end of the locating member, the location of the event can be calculated. The voltage drop can conveniently be measured by means of a third conductor which connects, via a high impedance a voltmeter (14), the ends of the locating member.