test equipment - What is a "universal bridge"?

I recently came across this intriguing instrument from the 1970s(?):

It's a "Universal Bridge" made by Wayne Kerr.

It's not a type of device I've come across before and I'm curious what these are (were?) used for.

Answer

User Janka actually has told the essentials in a short form. Computerized measuring equipment weren't generally available in 1970, they started to appear after the microprocessors were common. Precision RLC and voltage measurements have been needed at least 150 years.

The grandgrandfather of this is Wheatstone's resistance bridge. The idea in bridges isn't to measure directly, but by comparing to a reference and finding the ratio between the reference and the unknown value.

When the object to be measured is far from ideal, for ex. a lossy inductor, quite complex solutions are needed. You have one. It finds at the same time the inductance and the loss resistance (or conductance) at certain operating frequency (audio range only to keep the stray capacitance and inductance out of the results). Or actually you find it. You turn the knobs until the meter is at zero position. You have "balanced the bridge".

In the 1980's instrument's like this were computerized. The user only connected the part to be measured, listened a moment how the relays clicked instead of turning the manual switches and red the results from the display (R and L)