For those of you more familiar with mices, the tracker ball is operated by rolling around the big ball in the centre, which controls the pointer. So the device itself stays put and you move the ball to navigate around the screen. There are three buttons above the ball, but only the first two are used by the Domesday System.

This tracker ball is branded "MEDL", which stands for Marconi Electronic Devices Limited.

It connects to the BBC user port via a standard 20-way IDC connector as can be seen in the picture.

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