Electrochemical experiments, using nickel cathodes in light water solutions, were used to examine the enthalpy generated by electrically driving each electrode pair compared with ohmic controls contained within the same solution. For nickel wire cathodes, the peak power amplification (πNi) was in the range of 1.44±0.58. For spiral-wound nickel cathodes with platinum foil anodes, πNi was 2.27±1.02. By contrast, neither iron nor aluminum cathodes demonstrated excess heat. Driving these nickel samples beyond several volts, however, produced an exponential falloff of the power gain. This biphasic response to increasing input power may be consistent with the quasi-one-dimensional model of isotope loading and may contribute to the difficulty of reproducing these phenomena.