Depends on where the ammeter is situated. Basically it measures the current flowing in that part of the circuit, in Amps or parts thereof e.g milliamps. It should always be connected in series (voltmeters always in parralell) with careful consideration regarding polarity in a DC circuit.
It makes sense to connect the ammeter nearest the power source e.g. the battery so that it will measure all loads (devices that use your battery for electrical supply).
Rough (basic) guide to interpreting the readings:
With all items of equipment turned off there should be no current flowing = no reading. If you do measure any current then suspect a fault or an unknown load somewhere.
Work out the 'normal' current draw for each item of equipment (check rating plates or use ohms law to find out) and compare each item of equipment individually with the ammeter reading against what it is meant to draw. The same = OK, Less than forcast = it's very efficient, don't complain, more than forecast = there's likely to be something wrong with it.
Very roughly and as an example, If you draw 5 amps for 1 hour on a 20 Amp-Hour (Ah) Battery then you have 3 hours left before you end up with a flat battery. (20 Amps/5 Amps = 4hours)
Don't be surprised by large initial momentary readings when you switch the load(s) on before settling down to a stready reading. This is due to 'current surge' that occurs when switching things on. (One reason why fuses and miniature circuit breakers etc are current rated above the load current rating otherwise they'd keep poping or tripping when you switched things on.)
Now if you were to add a voltmeter to the circuit you would get a whole new dimension on the health of the circuitry. By monitoring and combining volts with amps you'd be able to work out what is happening with the circuit(s) much better than just monitoring current alone. In other words you're only getting one half of the story.
It seems to me that a reasonable quality 'multimeter' (about £30 and it don't matter if it's digital or analogue) with ranges in DC volts, DC amps and ohms is a good bit of kit to have aboard. You can pick on any accessable part of the circuit and do spot checks to monitor the health of the system or indeed to assist in fault finding. (Great bit of kit around the car as well.)
Of course (getting back to ammeters), an ammeter connected between the charging circuit and the battery will give an indication of how much the battery is drawing in amps whilst it is charging and this can help indicate the health of the battery. The higher the current on first charging a battery the lower the charge in the battery to start with. As the battery takes on charge and charges up, it will (should) if healthy, reduce its current draw (the amps reading reduces). I hasten to add I talk about lead/acid batteries here.
To have good all round monitoring then you'd need a voltmeter connected across the battery (The Battery Volts), an ammeter between the battery and all your loads (The Load Current) and an ammeter between the charger (e.g. dynamo/alternator output or mains battery charger) and the battery (The Charge Current). The voltmeter will draw a little current itself and so should be able to be isolated to prevent the battery from discharging through the voltmeter over a long period of time without regular recharging.
<P ID="edit"><FONT SIZE=-1>Edited by RON on Mon Mar 25 21:32:35 2002 (server time).</FONT></P>
I agree with all tht Ron has posted.
One further point though. To give meaningful indications the ammeter rating will probably be modest, say 50-0-50 amps. Pehaps a little less.
Whatever you do in incorporating an ammeter in your circuitry, DO NOT allow the starter motor current to flow through the ammeter.
The starter draws a couple of hundred amps at switch-on, which will destroy the ammeter.
Best of luck, Rod.
