One detects warmth of love in the jolly twosome (12)

I believe the answer is:

thermocouple

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'one detects warmth of love in' is the definition.
I can't judge whether this defines the answer.

'jolly twosome' is the wordplay.
'jolly' becomes 'thermo' (I can't explain this - if you can you should believe this answer much more).
'twosome' becomes 'couple' (synonyms).
'thermo'+'couple'='THERMOCOUPLE'

'the' acts as a link.

Can you help me to learn more?

(Another definition for thermocouple that I've seen is " It measures heat".)

I've seen this clue in The Times and The Sunday Times.

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