The mercury rectifier was used well into the 1970s, when it was finally replaced by semiconductor rectifiers.

Semiconductor rectifiers.

A metal rectifier is an early type of semiconductor rectifier in which the semiconductor is copper oxide or selenium.

Mercury-arc rectifiers have been replaced by silicon semiconductor rectifiers and high-power thyristor circuits in the mid 1970s.

Before the development of silicon semiconductor rectifiers, vacuum tube thermionic diodes and copper oxide- or selenium-based metal rectifier stacks were used.

Before the invention of mercury arc rectifiers and high-power semiconductor rectifiers, this conversion could only be accomplished using motor-generators or rotary converters.

The principle of operation of a metal rectifier is related to modern semiconductor rectifiers (Schottky diodes and p-n diodes), but somewhat more complex.

Originally they used rotary converters, a few of which are even still in operation, but most were supplanted first by mercury arc rectifiers and then by semiconductor rectifiers.

AC to DC synchronous rotary converters were made obsolete by mercury arc rectifiers in the 1930s and later on by semiconductor rectifiers in the 1960s.

They were the primary method of high power rectification before the advent of semiconductor rectifiers, such as diodes, thyristors and gate turn-off thyristors (GTOs) in the 1970s.

Dynamos still have some uses in low power applications, particularly where low voltage DC is required, since an alternator with a semiconductor rectifier can be inefficient in these applications.

These low frequencies were later made completely unnecessary by high power locomotive rectifiers that can convert any AC frequency to DC: first the mercury-arc rectifier and then the semiconductor rectifier.