Abstract: A pyrometer has a lock nut that engages a threaded sighting tube. The lock nut encloses a ratchet mechanism formed by two coaxial collars with inclined ratchet teeth at their ends. The teeth are urged together by a helical spring, one of the collars being prevented from rotation relative to the nut by means of a pin that engages a slot in the collar. The other collar has an annular face which is urged by a disc spring against a washer locked with the body of the pyrometer so as to form a friction clutch. The ratchet allows the nut to be tightened but it can only be untightened by overcoming the friction of the clutch.