Mercury was an online rotor machine descended from Typex, but modified to achieve a longer cycle length using a so-called double-drum basket system.
Mercury links were installed between the UK and various Overseas stations, including in Canada, Australia, Singapore, Cyprus, Germany, France, Middle East, Washington, Nairobi and Colombo.
A miniaturised version of Mercury was designed, named Ariel, but this machine appears not to have been adopted for operational use.
It had been mathematically determined that Typex had a sufficiently large cycle to permit only 750 characters to be sent using a single arrangement of its rotors without fear of compromising security.
A counter recorded the number of keystrokes and when these reached 750 a predetermined rotor was manually advanced one step, thus permitting TypeX to safely encrypt messages with more than 750 key strokes.