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This study concerns the measurement of electrothermal efficiency in an ac plasma torch. It was found that the best method of measuring arc input power was an electronic system involving multiplication and integration although thermal wattmeters could be used with a slight loss of accuracy. A self-jacketed parallel flow calorimeter was found best for measuring the heat content of the torch output. The flow-dependent thermal efficiency of this torch was found to exceed 60 percent. For established designs of torch the electrode losses can be used to establish the input power to the arc, and thus eliminate the need for an electrical measurement.