I found this wonderful old ammeter. Notice that there is a curved mirror strip below the number scale. In the top picture you can clearly see the reflection of the needle in the mirror. This means that I was not reading the meter from directly above the needle. That's called a parallax error and in this case it means that the ammeter looks like it has a positive reading with nothing connected. Note also the non-linear scale. It is clearly not designed to make readings below 5A. Each scale division above 5A is worth 0.5A but these scale divisions are more compressed above 25A.
