Question

state what happen to the ammeter reading if rheostat is replaced with a short copper wire

Answer 1

Right now, the rheostat is the ONLY resistance in the circuit.  If it's replaced with a short copper wire, then there's no resistance at all between the terminals of the battery.  

As soon as you go to close the switch:

-- There's a big spark at the switch.

-- There's an audible "click", as the needle of the Ammeter slams into the peg  at the top of its scale.  (It's trying to read infinite, or very very large, current.)

-- The meter could burn out very quickly.  If it doesn't, then

-- Somewhere between the battery and the Ammeter, or between the Ammeter and the switch, or between the switch and the battery, the WIRE will get very hot, and try to behave like an electric toaster.  If the wire is bare, you'll see it glow red.  If the wire has plastic or rubber insulation on it, the insulation will smoke.

-- The batteries will get hot, and quickly die.

Answer 2

When the rheostat is replaced with a short copper wire ; The ammeter reading would try to record an Infinite/very large current

The rheostat seen in the circuit acts as a resistance between the terminals of the battery therefore when replaced with a copper wire ( conductor ) the flow of current from one terminal to another will be without any form of resistance ( infinite current ).

When the switch is closed after replacing the rheostat with a copper wire the switch will experience a spark with the needle of the Ammeter slamming into the peg at the top of the ammeter scale. And this is because the ammeter tries to record an infinite/ very large amount of current flow in the circuit.

 

Hence we can conclude that When the rheostat is replaced with a short copper wire ;The ammeter reading would try to record an Infinite/very large current.

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