We break into the path of the electric current and interpose the meter like a tollbooth on a road, g/5. There is still a complete circuit, and as far as the battery and bulb are concerned, the ammeter is just another segment of wire.
Does it matter where in the circuit we place the ammeter? Could we, for instance, have put it in the left side of the circuit instead of the right? Conservation of charge tells us that this can make no difference. Charge is not destroyed or “used up” by the light bulb, so we will get the same current reading on either side of it. What is “used up” is energy stored in the battery, which is being converted into heat and light energy.