The ammeter is likely to burn out when connected in parallel due to the following reasons:

1. Current Division:
When an ammeter is connected in parallel, it is subjected to the entire current flowing through the circuit. The ammeter has a very low resistance, which is typically in the range of milliohms. As a result, the parallel combination of the ammeter and the circuit creates a low resistance path for the current to flow. This leads to a significant amount of current passing through the ammeter, causing it to burn out.

2. Overloading:
When the ammeter is connected in parallel, it is essentially short-circuited across the circuit. This means that the ammeter is directly connected across the voltage source, resulting in a high current flowing through it. The ammeter is designed to measure small currents, typically in the range of milliamperes. However, when connected in parallel, it is exposed to a much higher current, exceeding its maximum rating. This overloads the ammeter and causes it to burn out.

3. Lack of Current Regulation:
When an ammeter is connected in parallel, it does not regulate the current flowing through it. Unlike when connected in series, where the ammeter is inserted in the path of the current and restricts its flow, in parallel, the ammeter becomes a part of the current path. This lack of regulation allows the current to freely flow through the ammeter, leading to excessive current and subsequent burnout.

4. Heat Dissipation:
When a large current passes through the ammeter connected in parallel, it generates a significant amount of heat due to the resistance of the ammeter. The ammeter is not designed to dissipate this amount of heat efficiently, leading to a buildup of excessive heat. This heat can damage the internal components of the ammeter and eventually cause it to burn out.

Conclusion:
In conclusion, the ammeter is likely to burn out when connected in parallel due to current division, overloading, lack of current regulation, and inadequate heat dissipation. It is crucial to connect the ammeter correctly in series to ensure accurate measurements and prevent damage to the instrument.