A self-regulating porous heater regulates the temperature of fluid passing through it. This device includes at least one layer of a porous material which has a magnetic permeability inversely related to temperature over a given range. Due to conductor skin effects, changes in permeability cause changes in the depth of penetration of alternating current passing through the magnetic layer. Changes in the depth of penetration, in turn, result in changes in resistance--the less the depth the greater the resistance--and in I.sup.2 R Joule heat. The combined result of these effects is to provide more heat where the temperature of the fluid and the layer through which the fluid flows is below a specified temperature which is related to the Curie temperature of the layer. Significantly, by properly defining pore dimensions, the heater device can filter contaminants from a fluid as it flows therethrough. In a preferred mode, a porous heater device is included in a chemical reaction chamber. Fluids flowing through the chamber are closely controlled in temperature through each reaction zone by the heater device (through which the fluids flow) and by an optional cooling element. By defining the structure of the chamber and the rate of fluid flow, close control of other reaction parameters can also be achieved. Such close control permits greater specifity of reaction yield and greater safety in various exothermic reactions.