Does anyone have any knowledge about whether there should be a gasket between the face of the ammeter and the ring which holds the glass? It seems that without a gasket the needle would touch the glass thereby nullifying the operation.
There is no gasket,but there should be a metal spacer to hold glass off of needle.
Different makers of the ammeter held the glass in different ways but typically the front Bezel of the ammeter had a lip that curved back towards the glass to retain the glass at the front then the glass disc went against that. There then was generally a thin brass ring spacer or bracket depending on the maker that held the glass disc from behind against the bezel lip at the front. The pointer and meter face was next and on the large ammeter that was painted with the scale markings. That metal plate with markings on it is generally called the meter "pan" and it is fastened to the movement so that the movement needle always has a certain spacing in front of the pan. The movement and the shunt are mounted to the case usually by shoulder washers to the rear case part of the meter. These mounting bolts are generally brass and are the + and - electrical connections to the ammeter. The factory drawings for the large meter show it as being accurate to within +/- 1 Amp at Full scale which is 20 amps so that equates to about +/- 5% accuracy. The later smaller ammeter started out with the same spec but that was very soon changed to +/- 1 amp at the 10 amp point on the scale so that would be only 10% accurate at 10 amp reading and does not guarantee it to be then +/- 2 amps at full scale and clearly I have seen some that are not accurate at all beyond the smaller part of the scale. But they ARE cheap and that is what T guys value most
Jack, I am finding that the molded metal ring that holds the glass in the outer mounting frame is not thick enough to allow for movement in the face if the face is placed directly on the metal ring without a gasket or spacer of some kind, hence my original question.
Which ammeter? I have seen paper in the large type, none that I can recall in the small ones.
We are looking at a Nagel Company with Ford Script large style.
Here is a large '21-'25 style Nagel brand ammeter with Ford script.
Taken apart just has the typical steel ring that spaces the glass lens from the black face of the instrument.
The glass lens is in the outer face ring, and the steel un-finished spacer ring lays in the outer face ring over the glass lens and rests against the steel case of the ammeter, holding the black meter face up and away.
The only paper gaskets I have seen are in the 'Hoyt' brand ammeters. But they too have a brass spacer ring, the very thin paper gasket works as some sort of a seal to the glass for moisture protection I guess.