Aquarium water heater

Claim 1

An immersion-type electric aquarium heater comprising: a. an electric heater element; b. temperature sensing means located at a spaced point from said heater element and thermally insulated therefrom and comprising a negative temperature coefficient thermistor, the change in electrical resistance exhibited by said thermistor over the temperature range of about 72* to about 86*F. being substantially linear with respect to variations in temperature over said temperature range; c. alternating current supply means for said heater element; d. heater element control means electrically connected to said alternating current supply means, said heater element and said temperature sensing means and comprising (i) a zero voltage switch, (ii) a bi-directional thyristor and (iii) a multi-turn potentiometer, said temperature sensing means and potentiometer connected in series in a circuit that is adapted to provide a voltage V to said zero voltage switch, said zero voltage switch operable responsive to said voltage V and adapted to electrically actuate said thyristor, said thyristor adapted to permit or halt the flow of alternating current to said heater element; and e. housing means formed of an electrical insulating material and adapted to receive said heater element and heater element control means and be positioned within an aquarium.