A laboratory thermometer is not used to measure human body temperature because its temperature range is too wide and not precise enough for the narrow range of human body temperature. Laboratory thermometers typically measure temperatures from -10°C to 110°C, while human body temperature is usually between 35°C and 42°C. This large range makes them less accurate for measuring small temperature variations within the body. Additionally, laboratory thermometers often lack the kink present in clinical thermometers, which is essential for preventing the mercury from falling back down and ensuring an accurate reading.