| http://www.w3.org/ns/prov#value | - Opposite ends of the heating element 18 are connected to correspondingly opposite terminals of a voltage source 6 which is preferably a regulated electrical power source, either alternating current or direct current, and preferably regulated both as to voltage and current to assure a constant power dissipation in element 18, regardless of aging or other effects which may result otherwise in changi
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