The jocular noun foolometer denotes a standard or device for the measurement of foolishness or folly.

This noun is from:
– the noun fool;
– the combining form -ometer, used to form nouns denoting an instrument for measuring something, and, informally, nouns denoting a measure of a quality, emotion, etc.

The Oxford English Dictionary (online edition, March 2022) has misdated a quotation from The Home and Foreign Review [cf. footnote]. It therefore appears that the noun foolometer was coined by the British Anglican cleric, author and wit Sydney Smith (1771-1845) in Second Letter to Archdeacon Singleton, Being the Third of the Cathedral Letters (London: Longman, Orme, Brown, Green, and Longmans, 1838):

I am astonished that these Ministers neglect the common precaution of a foolometer, with which no public man should be unprovided: I mean, the acquaintance and society of three or four regular British fools as a test of public opinion. Every Cabinet Minister should judge of all his measures by his foolometer, as a navigator crowds or shortens sail by the barometer in his cabin. I have a very valuable instrument of that kind myself, which I have used for many years; and I would be bound to predict, with the utmost nicety, by the help of this machine, the precise effect which any measure would produce upon public opinion.

From May 1838 onwards, several British newspapers reprinted the above-quoted passage from Sydney Smith’s Second Letter to Archdeacon Singleton, thus popularising the noun foolometer. This popularity can be seen in the following from a correspondence from London, published in The Scotsman, or, Edinburgh Political and Literary Journal (Edinburgh, Midlothian, Scotland) of Saturday 9^th June 1838:

The more I think of all that I see and hear around me, the more I am impressed with the belief that, in as far as the Liberal party in the present House of Commons is concerned, our reign is over—our moving power gone. There has long been symptoms of a tendency on the part of the pure Whigs to ally themselves with the Tories, rather than with the moderate Radicals, to whose persevering and most disinterested support they owe so much. Mr Fazakerley’s vote on Friday is a proof of the extent to which this feeling is already carried; and of the importance with which he, and many of his friends, look forward to the period when the restraint they are now enduring will be at an end. Mr Fazakerley voted against Lord John Russell in favour of the £10 qualification; and although individually his opinion may be of little importance, it is of importance as arguing that of others, who have as yet had the discretion to keep their intentions to themselves. Mr Fazakerley is an admirable—I will not say Foolometer, (with Sydney Smith,) but Old-Whigometer, if you will allow me to coin the phrase. Whatever he says or does, you may be pretty sure that there are some ten or a dozen old Whig county members ready to do and say, as soon as the opportunity serves; and I consequently regard his conduct on Friday as indicating not merely the line to be taken by these gentlemen, but that which the Government wishes them to take under certain circumstances and at a proper time.

The earliest occurrence that I have found of foolometer used without explicit reference to Sydney Smith or to his text is from The Albion (Liverpool, Lancashire, England) of Monday 25^th February 1839—the following was written in reaction to an article published in the Mail, referred to as “our contemporary”:

The Town Council is certainly obnoxious to Tory feeling, but the Watch Committee ought to be exempt from Conservative vituperation. About a moiety of the members have always been regarded as good Tories, and it is hardly polite to call them fools: it is the first time that Mr. Thomas Case or Mr. Nelson Wood was ever so designated. The occasion hardly warranted such strong language; for Mr. Hall is not likely to be influenced by the “foolometer” of our contemporary.

Note: There is an error in the Oxford English Dictionary (online edition, March 2022). This dictionary states that the following quotation is from The Home and Foreign Review (London, England) of October 1836, whereas it is from George Eliot’s Novels, by the British literary critic Richard Simpson (1820-1876), published in The Home and Foreign Review (London, England) of October 1863:
—Context: Richard Simpson was referring to Romola, by the British novelist George Eliot (Mary Ann Evans – 1819-1880), first published in serial form, from July 1862 to August 1863, in The Cornhill Magazine (London, England), and first published in book form in 1863:

Romola is infinitely stronger than Tito, or Fra Girolamo himself, though she owes every thing to his opportune influence; Tessa is a foolometer, showing how little womanhood is wanted to balance all the manly intelligence and will of Tito.