A time domain reflectometer (TDR) is similar in principle to radar for detecting discontinuities, breaks, and other anomalies in metallic cables such as twisted pair and coax.  The TDR transmits a step pulse in the cable and listens for a reflection.  If the cable is properly terminated then the pulse will be fully absorbed by the termination and there will be no reflection.  Any discontinuity will cause a reflection which is added or subtracted distorting the step pulse.  The round trip time can then be measured and using the speed of signal propagation in the cable the discontinuity can be pinpointed with extreme accuracy.

Besides use in locating underground or overground cable faults, TDRs were essential for finding faults in the miles of cable in an airplane or jet.  One specific instance of the value of a TDR was one of the museum volunteers helping an friend find a short in a water well power line which ran out 2000 feet from the house.  They disconnected the power from the breaker panel and connected it to the TDR.  They had to guess at the propagation velocity and dialed out 380 feet to the short as indicated on the TDR. They went and paced off 380 feet and about 6 feet away was a post the owner had put in the ground a couple of years ago. That post cleaned some isolation off of that cable and caused the eventual failure. The TDR found the issue in nothing flat!

The museum has a 1502 portable TDR cable tester on display.  The 1502 is a small lightweight portable cable tester that was introduced in 1976 for a price of $3360 including the optional paper recorder.The 1502 can resolve to 0.6 inches at distances up to 2000 feet.  In the 1502 on display at the museum in this photo the cable is unterminated and the reflection is adding to the step pulse.  The cable dielectric buttons select the type of cable (which sets the appropriate velocity factor) and the zero reference is adjusted to align the leading edge of the step pulse to the left side of the screen.

The dial indicator is then adjusted to align the discontinuity to the same edge and the distance to the fault can be read directly from the dial indicator (56) as seen in this photo.  You can also see another smaller discontinuity in the trace as the signal is again reflected.

We sometimes use TDR for our STEM demonstrations where we calculate the speed of propagation in the demonstration cable at 124,827 miles/second.

This page is from the 1976 catalog on TDR cable testers.