Abstract: The invention provides a fire detection system which comprises an electric thermal transducer such as a thermistor placed in a turbulent gas stream. The transducer produces a signal, having an AC and a DC component, which signal is preferably amplified. The components are separated out in a discriminator, for instance an AC amplifier and the AC component is squared and integrated to give a final output which is proportional to the temperature of a heat source, such as a fire, upstream of the transducer. The invention also provides a fire detection system comprising a series of transducers connected to a single processing apparatus, the signal from the transducers being compared to locate a heat source. The invention also provides a method of detecting a fire using a system as described above.