Which instrument is used to measure the length of a cable?

The time domain reflectometer (TDR) is an instrument specifically designed to measure the length of a cable. It operates by sending a signal down the cable and analyzing the reflections that occur when the signal encounters changes in impedance along the cable's length. This allows the TDR to determine the precise length of the cable by calculating the time it takes for the reflections to return to the device.

While optical time-domain reflectometers (OTDR) share similar principles for measuring optical fibers, they are tailored for fiber optic cables rather than general cable length measurement. Cable length testers do measure length, but they typically do so using simpler methods and may not provide as comprehensive information as a TDR. A digital multimeter is primarily used for measuring voltage, current, and resistance, and it is not designed for measuring cable lengths. Each of these instruments has its specialized applications, but the TDR stands out for accurately measuring cable lengths through its time-domain reflection methodology.