AG Thornton was established as a manufacture of drawing instruements when it took over the Precision Instrument Co in 1901. This merger added a range of PIC slide rules to their range. the plastic slide rules where introduced in the 1960s and in 1967 the company became British Thornton.
The slide rule was the evelution of Napier's Rods (or bones) which where used for multiplication. Before the electronic calculator, in 1974, the slide rule was used by scientists and students to perform calculations in mathematics and physics. The alternative where 4 and 5 figure logarithm tables. The 10" slide rule shown here (P271 Log Log) had an accuracy of 3 decimal places. It's operation was efficient and the Pickett-brand slide rules were carried on five Apollo space missions, including to the moon.
I used mine for CSE examinations through A Level Mathematics and physic and at university. It enabled me to undertake more experiments in physics than my fellow students and I could finish writing up the notes and calculations when they where still packing away.