Why is it necessary to consider positive and negative terminals of the ammeter when you connect leads from it to the circuit?

When you are dealing with DC voltages polarity is very important. Inside the ammeter is a coil through which the circuit amperage flows. Inside this coil is connected a steel armature of the meter movement. The more current is applied through the coil, the more the steel of the meter movement is drawn into the coil and this in turn moves the needle on the face of the meter to move further up the scale. If the coil polarity is reversed the magnetic field in the coil is reversed and the steel armature is driven in reverse. So instead of the meter movement moving up the scale it is driven to the zero end of the meter scale and it hammers up against the stop pin on the scale face.