Please excuse my butting in ... I'm a stranger here and don't want to act like a know-it-all. That said, I'm gonna plunge in anyway... [img]images/graemlins/grin.gif[/img]
I went away yesterday thinking that RRich's reply didn't do the job, and I was short on time to reply. The ammeter is
in series with the load. The load is the charging circuit (alternator) in parallel with the lights, horn etc. I searched for an image that may help.
This ammeter uses an external shunt (instead of the shunt being inside the ammeter case) but the principle is the same. The ammeter measures the differential current between the battery and the load. Ok, so the starter and battery are on one side, and the alternator and the rest of the car are on the other.
I think that RRich wanted to point out that, under typical driving conditions, there is no differential current between the battery and the load because the alternator supplies all the current needed by the load. If the engine speed drops or stops, or if the battery is low, or the alternator fails, current flows through the ammeter. The +/- direction of the ammeter needle indicates whether the battery or the load is calling for more current.
Are ammeter's dangerous? RRich is right - under normal operating conditions, no. However, I did find a reference that said no vehicle equipped with more than an 85A alternator should run an ammeter. You can imagine that a car with a 105A alternator might draw too much current for the ammeter at idle, or if the alternator fails, or if the battery is kaput.
So, my own take on this is that voltmeter will tell you basically the same thing as an ammeter with more safety - but that's a different thread.
best regards [img]images/graemlins/cool.gif[/img] Tim