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This exhibit is a large multi-phase mercury-arc rectifier. The maker/ trade name is Nevelin. Such rectifiers were used industry to convert incoming three phase AC mains to DC for running older equipment that employed DC motors. Printing presses were one such application.

The envelope contains the words Nevelin Croydon England. In use the bell is filled with a bright violet glow. Lovely to look at but full of UV and so normally these rectifiers were operated in sealed cages with shields to block the light.

The six anodes are arranged around the central pool of mercury. A starter electrode, controlled by a solenoid, initiates the arc. In use the rectifier drops approximately 20 Volts for a wide range of currents. If the current falls below the minimum holding current the arc will be extinguished.

The following has been taken from the electrokinetica.org web site and relates to a similar rectifier.

In total, 12 electrodes are fitted to this bulb: 3 main anodes, 3 control grids, 2 auxiliary anodes, 2 excitation anodes, ignition dipper and cathode.The three main anodes are housed in angled side-arms; this protects against reverse current flow that would result from ionic bombardment if the anodes were in line-of-sight of the cathode pool. Also in the main arms are the control grids, connected via small terminals projecting from the side. The excitation anodes are mounted conventionally in small arms near the base of the bulb to keep the forward voltage low. Nevelin usually arranged the cathodes of their dipper-ignition rectifiers to lead-in through a side-arm.

Two of the six anodes.

The base showing the mercury pool and the cathode connection on the left. The smaller electrodes are the starting anodes. The coils below them are the solenoids that attract the flexible arm of the starting anode. With the solenoids energised the flexible arm will vibrate at mains frequency and periodically touch the mercury causing a small patch of ions. In time when sufficient mercury has ionised these ions reach the main anodes and the full arc strikes.

Thanks to Mario Maqana Canteli for sending these pictures.

The balloon envelope is  mm in diameter, and excluding the base pins is  mm tall.

Reference: Data-sheet.

Absolute Maximum Operating Conditions^¶

PDF scanned from an original document held by the museum