<p>An automatic instrument for recording the percentage of carbon dioxide in air supplied to greenhouses for experimental work is described. The instrument gives a record over the range of 0–3.5% carbon dioxide. The application of the thermal conductivity method of gas analysis is described, and several new features not heretofore found in such apparatus are discussed.</p><p>Convection Type Thermal Conductivity Cell.—A cell construction is described which renders the indications of gas concentration independent of the velocity of flow through the sampling line. The circulation of gas through the cell is dependent on the platinum wire temperature only.</p><p>Compensation for Variations in Bridge Current.—A method is described for compensating the thermal gas analysis Wheatstone bridge for variations in the bridge current by introducing a manganin resistance in series with one of the cell, wires.</p><p>Compensation for Variations in Cell Temperature.—A method is described for compensating the bridge for variations in cell temperature by introducing a nickel resistance in one of the bridge arms.</p><p>The results obtained with this instrument indicate that it is accurate to 0.05% of carbon dioxide, over the range of 0–3.5% carbon dioxide. The application of similar gas analysis bridges and recorders to the analysis of other gas mixtures where the same range of thermal conductivity differences is involved is discussed.</p>

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