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A single-unit mercury arc rectifier of 3,000-kw rating at 600 volts is discussed as regards design and operation. A typical installation, that of the 8th Avenue subway of the New York Board of Transportation, is described where units of this size have been advantageously applied to the sub-surface substations. An important characteristic of the rectifier unit is its operating temperature which is materially higher than heretofore used in general practise. Tests show that up to a certain temperature the arc drop is reduced as the temperature is raised. This indicates that the maximum efficiency is obtained by operating at a high temperature. Among other advantages of a high operating temperature is the increased cooling efficiency. The rectifier is of the thyratron type with insulated grids surrounding the anodes. In addition to straight rectification, it is expected that it will find many applications in the future where thyratron control is desired.