Electrical current flow is induced in a ferromagnetic conductor providing time-varying electrical current at a first frequency to an electrical conductor located in a formation. The ferromagnetic conductor at least partially surrounds and at least partially extends lengthwise around the electrical conductor. The ferromagnetic conductor resistively heats up to a first temperature of at most about 300° C. Water in the formation is vaporized with heat at the first temperature. Subsequently, time-varying electrical current at a second frequency is provided to the elongated electrical conductor to induce electrical current flow at the second frequency such that the ferromagnetic conductor resistively heats up to a second temperature above about 300° C. Heat transfers from the ferromagnetic conductor at the second temperature to at least a part of the formation to mobilize at least some hydrocarbons in the part of the formation.