The Melz LP-4 is a bizarre Soviet-produced linear trochotron in a large acorn-style envelope. This tube, designed for base-10 counting, is based on a 1946 design for a linear magnetic counting tube originally patented by Ericsson. Though Ericsson produced several prototypes of it's linear magnetic counting tube, it appears to have never put the tube into production. Unlike the Ericsson prototypes, which had electrodes arranged in horizontally within a tubular envelope, the LP4 is pseudo-circular, with its linear electrode array curved into an incomplete ring to facilitate installation within a compact cylindrical envelope. When installed in a device, the LP4 must be mated with an external magnet; it is the interaction of the electron beam with this magnet that allows the beam to be 'bumped' from electrode to electrode with an external pulse.
Ericsson did eventually produce a trochotron magnetic beam switching tube commercially; the RYG10. The RYG10 is a circular beam switching tube, and includes phosphor-coated targets that allow it to display it's own count.
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