(1) Since the ammeter cannot be used beyond its range, when the ammeter reaches full scale, the current in the circuit is 100mA=0.1A;

At this time, the circuit The resistance in is: R=UI=3V0.1A=30Ω;

As can be seen from the image, when the resistance of the thermistor is 30Ω, the temperature is 25℃.

Answer: The maximum temperature that the thermometer can measure is 25℃.

(2) From the above analysis, it can be seen that when the temperature reaches 25℃, if the temperature continues to rise, the resistance of the thermistor will decrease. Since only the thermistor is connected in the circuit, the circuit will The current in the meter increases, exceeding the range of the ammeter and damaging the ammeter. Therefore, the circuit designed by Li Ming is inappropriate.

(3) It can be seen from the image that when the temperature is 80°C and the resistance of the thermistor is 5Ω,

In order to expand the range of the temperature meter to 80°C, that is When the temperature reaches 80°C, the current in the circuit cannot exceed 0.1A, and because the resistance of the thermistor is 5Ω at this time, a resistor is connected in series in the circuit, as shown in the figure:

When the current expression reaches full scale When , the resistance in the circuit is 30Ω, that is, R total = 30Ω.

So the series resistance is R2=Rtotal-R1=30Ω-5Ω=25Ω.

Answer: The resistance of the new resistor is 25Ω.

(4) It can be seen from the image: at 0℃, the resistance of the thermistor is 100Ω. At this time, the total current of the circuit is: I=UR total=3V100Ω+25Ω=0.024A=24mA ;

Answer: The 0℃ on the temperature meter should correspond to the 24mA position on the ammeter dial.