The strength of the electromagnet is directly proportional to the amount of current flowing in the wire. Thus the rotary displacement of the bar is directly proportional to the amount of current flowing, at least through ninety degrees of rotation. Note that this device measures current flowing through it. It does not directly measure voltage. Typical full-range current of a d'Arsonval meter movement is 50 microamps.
Ammeters go in series in the circuit they are measuring.
Meters are sometimes distinguished by how the axle on which the needle rides is mounted in the instrument. They are used because they are very hard and don't wear out quickly. Sometimes there is a spring behind one of the pivot jewels to keep the whole assembly together even if it is dropped. The other common arrangement is to make the axle a ribbon, rather than a round bar, and stretch it between two mounting blocks. It's a bit more fragile than jewelled pivots, but can be more sensitive as well.