Question

Some time after the heater is on the ammeter Reading seem to decrease suggest why this happen

Answer 1

This might be when the resistor with the positive temperature coefficient (PTC) is connected to the constant voltage source with voltage $V$. Then, after the heater is on, the resistance $R$ of the resistor increases, because of PTC. Thus, according to the Ohm’s law:

$$I = \dfrac{V}{R}$$

the current through the resistor should decrease, causing the decreasing of the ammeter reading.