For non ideal ammeter and voltmeter, ammeter have low resistance and voltmeter have  high resistance. Therefore the main current in the circuit will be very low and almost all  current will flow through the ammeter. It emf of cell is very high then current in ammeter  is very high result of this current the devices may get damaged. If devices are ideal that  means resistance of voltmeter is infinity. so that current in the circuit is zero. Therefore  ammeter will read zero reading and voltmeter will read the emf of cell.