Complex sentence: They may work by contact (the electronic sensor is placed in the location whose temperature is to be measured, and left long enough to reach equilibrium) or by remote sensing: an infrared sensor responds to the radiation spectrum emitted from the location.
Split into simpler sentences: They may work by contact (the electronic sensor is placed in the location whose temperature is to be measured, and left long enough to reach equilibrium). Other electronic thermometers work by remote sensing: an infrared sensor responds to the radiation spectrum emitted from the location.