Thermometer are useless because it shows its own reading? Please someone answer me how.
I am not sure I understand you question.
Are you asking how we calibrate thermometers?
I.e. how to relate what a thermometer shows to the thermodynamic temperature.
Yeah, it's a badly worded question. Are you suggesting that the thermometer reads only its own temperature? If so, then you are correct; but... its own temperature is determined by its environment, so it gives an accurate account of what it's measuring.
Any instrument can be useless. It all depends on their proper usage and calibration. Thermometers are obviously not useless when used within their devised usage. Again, please restate your question.
Thermometers are designed to quickly achieve thermal equilibrium with their surroundings, thus reading the temperature of the surrounding.
Separate names with a comma.