Way back in the 1970's I visited the National Physical Laboratory (NPL) at
Teddington, West London.

I was priviledged to see, but not to touch, the National Standards of the
Ohm and of the Volt. There were a number of 4-terminal resistors submerged
in a temperature-controlled oil bath. There was about a dozen Western Cells
with connecting wires submerged in another temperature-controlled oil bath.
There were small fans in the oil to ensure a very slow circulation.

I asked my guide, the physicist responsible for maintaining these vital
standards of measurement on which the Commercial, Industrial, Engineering,
Scientific life of the Nation is supported, what special sort of oil was
used in the baths?

He said it was the same stuff, without the perfume, as young men used for
hair oil and came from a tree in Malaya.
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Reg, G4FGQ