This wrist calculator was sold as a kit, costing £9.95. A three-position button provided access to memory and square root functions.

Although futuristic, the wearable tech was beset by problems. Running on six hearing-aid batteries, these only lasted about 90 minutes and you then had to disassemble the unit to change them. The calculator was also unreliable. Unsurprisingly, sales were poor, and many of the units purchased were returned. 

The calculator was produced by Sinclair Instrument, a parallel company to Sinclair Radionics.