When laying out the design of ah alternator that will be used to transmit power over a long transmission line, it is necessary to consider several factors in addition to the specified full-load rating before one can decide on the relative electric and magnetic loadings of the machine, or even the frame size on which it should be built.In attempting a survey of the problem, the paper was divided into six sections, which may be summarized as follows :—(1) General discussion of the possible instability of an alternator connected to a transmission line.(2) Varying-voltage and constant-voltage ' transmission lines. (3) A more detailed consideration of the alternator, showing that it is possible to determine directly from its open-circuit and short-circuit characteristics the maximum leading kVA with which it can deal at normal voltage, for complete stability down to zero voltage.(4) Effect of the exciter on the stability of the generating unit.(5) Variations in station voltage to be expected when operating the high-tension line switches.(6) Particulars of an alternator built for a 50-period line demanding a charging current equivalent to 8 100 kVA at 11 000 volts.