Faraday to John Wilson Croker   8 May 1847¹

Royal Institution | 8 May 1847

My dear Sir

From your descriptions² I conclude that your three thermometers are inaccurately graduated at the low temperatures. This is very often the case with thermometers of ordinary construction & price. In the laboratory we are always obliged to verify an instrument before we can trust it. Thermometers profess to be graduated in relation to their scales so that no influence belonging to any difference of these should appear. If you put them all three into a vessel containing mixed ice & water they should all indicate the same degree & that 32˚ then whether you lower the temperature by adding salt - or raise it by adding warm water, they should all agree. But all this requires care in moving the thermometers in the liquid so that they may really have the same temperature and if they do not agree you have no means of picking out the accurate one (if such there be) except by it according with 32˚. Still the trials may interest you[.]

I am | My dear Sir | Very Truly Yours | M. Faraday

Right Honble | J.W. Croker Esq | &c &c &c