This turned the machine from a voltage-to-current amplifier into a voltage-to-voltage amplifier, and they called the new variant an Amplidyne.
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Trials began with a four-car formation on the District line between High Street Kensington and Putney Bridge in September 1937, and a six-car formation began working on the Hammersmith line in January 1938.
The metadyne equipment controlled four traction motors on two cars and allowed for regenerative braking, although an air braking was fitted for low speeds and if the traction supply was unreceptive to the regenerated current.
In the late 1920s/1930s 1500V DC began to be used on the PO rail system; the locomotives were converted for 1500V operation at workshops at Vitry using a Metadyne for traction control.