Abstract: An ionization-type fire detector having a pair of ionized chambers, one of which is arranged so that products of combustion can flow therethrough while the other is substantially isolated from the products of combustion. As a result, when combustion products are present, the electrical conductivity in one chamber is different than the other, and this condition is sensed to provide a fire alarm signal. The first-mentioned ionized chamber is defined by inner and outer shells of plastic material which are suitably spaced from each other and with the inner surface of the outer shell being provided with a conductive coating. The spaced plastic surfaces are curved, and one of the shells defines a cavity which forms at least a part of said first-mentioned chamber. It has been found that this minimizes the adverse effects caused by wind drafts which otherwise can affect the electrical conductivity in the first chamber and provide an erroneous fire alarm signal.