Maybe a previous owner had a secret stash of Bolivian Marching Powder.

Sorry, couldn't resist it!

email (option): sacombsashtrees@hotmail.com

email (option): meck04@bigpond.com

It could be the zinc or cad plating corroding off, its amazing how much bigger something appears when it oxidises.

Whatever it is try not to inhale it.

Rob

email (option): robmiller11(a)yahoo.co.uk

I've not been able to find any indication that Smiths motor vehicle instruments ever had luminescent dials. There are no thick lumpy painted marks on the silk-screened dials and in most applications they were back-lit so it would not have been necessary.

It might be a simple as lime scale from water ingress depending how it has been stored.

It is a little known fact that the blue smoke inside your electric wires tends to leak out and accumulate in the ammeter casing. Of course this smoke condenses and you will find a white powder residue, which is oxidised electricity. You will probably remember the "Solid State" transistor radios of the 1960s which were in fact powered by this same electricity oxide. However, it was found to be highly unstable when plunged in a bath of hot water and it is now thought that many mysterious bathroom deaths in the Sixties were caused not by drug misuse, but by young people accidentally knocking their tranny radio into the water while bathing.

email (option): fergusanckorn@icloud.com

Going by the price these ammeters are fetching I think it must be diamond dust... ...Ian

email (option): ian@wright52.plus.com

Are these ammeters made of brass, if so I have one kicking around in my garage with no glass and a broken needle although the remains seem to move side to side. Are these repairable?

think it might be oxidized brass from the main back casing. All cleaned out now but there was a fair amount of it.
No illuminous markings on the face. Just white painted on black background. That needle is VERY delicate!

email (option): meck04@bigpond.com