A hydrogen generator comprising: a reactant composition comprising a reactant that thermally decomposes to produce hydrogen gas when heated to at least a minimum temperature; and an electric circuit in electrical communication with an energy source, the electric circuit comprising: a plurality of parallel branches in a sequence, the sequence comprising in order a first branch and at least one next branch, the final at least one next branch being a last branch; a heating element in each of the parallel branches; a normally open switch between each pair of parallel branches in the sequence, the normally open switch having an open state and a closed state; and a normally closed switch in each parallel branch except the last branch, the normally open switch having a closed state and an open state; wherein the hydrogen generator is configured such that: current flowing through the circuit initially flows through only the first branch; the heating element in the first branch is disposed to heat a first portion of the reactant composition, a first normally open switch and a first normally closed switch; when the first normally open switch is in the closed state, a current flow through the circuit does not include a current flow through the adjacent next branch, in which a next heating element is disposed to heat a next portion of the reactant composition; when the first normally closed switch is in the open state, a current flow through the circuit does not include a current flow through the first branch; each of the normally open switches comprises a switch material with a resistivity that permanently decreases in response to heating to at least a first temperature such that the normally open switch is in the closed state upon heating to at least the first temperature; and each of the normally closed switches is in the open state upon heating to at least a second temperature. a reactant composition comprising a reactant that thermally decomposes to produce hydrogen gas when heated to at least a minimum temperature; and an electric circuit in electrical communication with an energy source, the electric circuit comprising: a plurality of parallel branches in a sequence, the sequence comprising in order a first branch and at least one next branch, the final at least one next branch being a last branch; a heating element in each of the parallel branches; a normally open switch between each pair of parallel branches in the sequence, the normally open switch having an open state and a closed state; and a normally closed switch in each parallel branch except the last branch, the normally open switch having a closed state and an open state; a plurality of parallel branches in a sequence, the sequence comprising in order a first branch and at least one next branch, the final at least one next branch being a last branch; a heating element in each of the parallel branches; a normally open switch between each pair of parallel branches in the sequence, the normally open switch having an open state and a closed state; and a normally closed switch in each parallel branch except the last branch, the normally open switch having a closed state and an open state; wherein the hydrogen generator is configured such that: current flowing through the circuit initially flows through only the first branch; the heating element in the first branch is disposed to heat a first portion of the reactant composition, a first normally open switch and a first normally closed switch; when the first normally open switch is in the closed state, a current flow through the circuit does not include a current flow through the adjacent next branch, in which a next heating element is disposed to heat a next portion of the reactant composition; when the first normally closed switch is in the open state, a current flow through the circuit does not include a current flow through the first branch; each of the normally open switches comprises a switch material with a resistivity that permanently decreases in response to heating to at least a first temperature such that the normally open switch is in the closed state upon heating to at least the first temperature; and each of the normally closed switches is in the open state upon heating to at least a second temperature. current flowing through the circuit initially flows through only the first branch; the heating element in the first branch is disposed to heat a first portion of the reactant composition, a first normally open switch and a first normally closed switch; when the first normally open switch is in the closed state, a current flow through the circuit does not include a current flow through the adjacent next branch, in which a next heating element is disposed to heat a next portion of the reactant composition; when the first normally closed switch is in the open state, a current flow through the circuit does not include a current flow through the first branch; each of the normally open switches comprises a switch material with a resistivity that permanently decreases in response to heating to at least a first temperature such that the normally open switch is in the closed state upon heating to at least the first temperature; and each of the normally closed switches is in the open state upon heating to at least a second temperature.