Degree Twaddell (°Tw)
Modern equivalent
Specific gravity = 1 + (°Tw ÷ 200)
A hydrometer graduation for liquids denser than water, devised by the Glasgow instrument maker William Twaddell.
The Twaddell scale solves a problem of mental arithmetic. Specific gravities cluster inconveniently between 1.000 and 2.000, which is awkward to read off a stem and worse to write down all day, so William Twaddell, an instrument maker in Glasgow, stretched the interval across two hundred divisions.
The conversion is therefore a single step: divide the degrees by two hundred and add one. Zero degrees Twaddell is pure water; a hundred and seventy degrees is sulphuric acid at a specific gravity of 1.85. A works chemist could call out a Twaddell reading and a foreman could act on it without touching a decimal point.
It was adopted across the British bleaching, dyeing, tanning and heavy chemical trades, where strong acids and alkalis had to be checked constantly and by people who were not chemists. That is its distinguishing feature: a scale designed around who would be reading it.
It measures only liquids denser than water. For anything lighter — spirits, oils — a different instrument was needed, which is why Britain ended up with a drawer full of hydrometers rather than one.