NOVELTY - A fuel cell comprises: (i) housing including anode chamber in communication with fuel source via conduit; (ii) cathode chamber in fluid communication with oxidizing agent; (iii) proton conducting membrane electrolyte separating the chambers; and (iv) heat-actuated valve proximate the conduit or a fuel concentration-actuated valve for automatically controlling flow of fuel from fuel source to anode chamber.    USE - Direct oxidation fuel cell, preferably a direct methanol fuel cell (claimed).    ADVANTAGE - The amount of fuel concentration in the fuel mixture of a direct methanol fuel cell is easily controlled.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (a) A method for controlling flow in a fuel cell by connecting the fuel cell to an electrical load, producing electrical energy, generating heat in response to the production of electricity by the fuel cell and automatically actuating a heat-sensitive valve for controlling the flow in response to the heat;    (b) A sensor for determining a concentration of fuel in a fuel mixture for a fuel cell comprising a conductor having individual portions positioned proximate one another within a first material, where the first material automatically expands upon exposure to a liquid medium;    (c) A method for determining a concentration of fuel in a fuel cell by providing a first material capable of expanding in response to a concentration of fuel in a fuel cell, where a conductor is positioned within the first material, flowing an electrical current through the conductor and measuring a resistance of the conductor, where as fuel concentration changes, the resistance of the conductor correspondingly changes; and    (d) A switch for a fuel cell comprising a heat actuated shape memory alloy where below a predetermined temperature, the switch is in a first position and upon the fuel cell reaching the predetermined temperature, the switch is switched to a second position.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-section of a fuel delivery conduit for a fuel cell system.    Shape memory alloy springs (50)    First end (52)    Second end (54)    Conduit wall (56)    Slight visible curve (58)