The rated voltage of the bulb is 2.5V, and the voltmeter range is 0 ~ 15 V, which is too large.
According to the current value in the title table, the ammeter should be in the range of 0 ~ 0.6A, which is too large.
(2) The brightness of the bulb is weak and the current is too small. According to I=UR, the current is small, the voltage may be small, and the resistance may be too large.
In the process of moving the slider, the resistance changes greatly, and the brightness of the two meters and the bulb changes little. It can be seen that the supply voltage is too low.
(3)①R = UI = 2.5v 0.24 a = 10.4ω。
② The resistances of the bulb with ②R = UI at 2.0V, 2.5V and 2.8V are10Ω,10.4Ω and10.8Ω respectively.
Therefore, the bulb voltage increases and the filament resistance increases.
The reason is that the bulb voltage, current, filament temperature and filament resistance increase.
(4) (1) Explore the relationship between the main circuit current and each branch current of the parallel circuit. The bulb and the sliding rheostat form a parallel circuit, with an ammeter on the main circuit and a switch on the bulb branch, as shown in the figure.
Close the switch, move the slider, the voltage of the sliding rheostat remains unchanged, the resistance changes, the current of the bulb branch remains unchanged, and observe the change of the main ammeter.
Keep the sliding vane of the sliding rheostat still, turn off the switch, keep the branch current of the sliding rheostat unchanged, reduce the bulb branch to zero, and observe the change of the main ammeter.
② During the experimental exploration, the sliding rheostat can not only change the current in the circuit, but also act as a resistor in the circuit. Cannot be 0 while moving the slider. If it is 0, the circuit will be short-circuited and the ammeter and power supply will be damaged.
So the answer is: (1)① There is no parallel voltmeter at both ends of the small bulb; ② The voltmeter range is too large (or the ammeter range is too large);
(2) The power supply voltage is too low;
(3)① 10.4Ω; ② Change; When the bulb emits light, the temperature rise causes the resistance change;
(4)① The circuit diagram is shown in the figure; ② Resistance; Resistance value cannot be zero.