A method for heating fluid, the method comprising: passing the fluid along a flow path from an inlet to an outlet, the flow path comprising at least first and second heating sections (116, 117, 118) positioned along the flow path such that fluid passing the first heating section subsequently passes the second heating section (117, 118), each heating section comprising at least one pair of electrodes (116a, 116b; 117a, 117b; 118a, 118b) between which an electric current is passed through the fluid to resistively heat the fluid during its passage along the flow path, and wherein at least one of said heating sections comprises at least one segmented electrode (116ai, 116aii, 116aiii; 117ai, 117aii, 117aiii; 118ai, 118aii, 118aiii), the segmented electrode comprising a plurality of electrically separable segments allowing an effective active area of the segmented electrode to be controlled by selectively activating the segments such that upon application of a voltage to the activated electrode segment(s), current drawn will depend upon the effective active area; measuring fluid conductivity at the inlet; determining from measured fluid conductivity a required voltage and current to be delivered to the fluid by the first heating section to raise the fluid temperature by a first desired amount; determining an altered fluid conductivity resulting from operation of the first heating section; determining from the altered fluid conductivity a required voltage and current to be delivered to the fluid by the second heating section to raise the fluid temperature by a second desired amount; and activating segments of the segmented electrode in a manner to effect delivery of desired current and voltage by the segmented electrode. passing the fluid along a flow path from an inlet to an outlet, the flow path comprising at least first and second heating sections (116, 117, 118) positioned along the flow path such that fluid passing the first heating section subsequently passes the second heating section (117, 118), each heating section comprising at least one pair of electrodes (116a, 116b; 117a, 117b; 118a, 118b) between which an electric current is passed through the fluid to resistively heat the fluid during its passage along the flow path, and wherein at least one of said heating sections comprises at least one segmented electrode (116ai, 116aii, 116aiii; 117ai, 117aii, 117aiii; 118ai, 118aii, 118aiii), the segmented electrode comprising a plurality of electrically separable segments allowing an effective active area of the segmented electrode to be controlled by selectively activating the segments such that upon application of a voltage to the activated electrode segment(s), current drawn will depend upon the effective active area; measuring fluid conductivity at the inlet; determining from measured fluid conductivity a required voltage and current to be delivered to the fluid by the first heating section to raise the fluid temperature by a first desired amount; determining an altered fluid conductivity resulting from operation of the first heating section; determining from the altered fluid conductivity a required voltage and current to be delivered to the fluid by the second heating section to raise the fluid temperature by a second desired amount; and activating segments of the segmented electrode in a manner to effect delivery of desired current and voltage by the segmented electrode.