In the manufacture of multiple glazing units, such as double glazing units, edge regions of at least one glass sheet are heated by passage of alternating electric current to weld the edge regions of adjacent sheets together. The current is passed continuously along two separate conducting paths through the glass, the currents in the two paths having an angular phase displacement relative to each other. The voltage used may be fixed and the phase displacement variable. Alternatively the phase displacement may be fixed and the voltage variable.
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