Correspondence

To the Editor: In the January 1924 issue of the JOURNAL, Mr. R. H. Marvin described a temperature and pressure correction chart for the sphere gap. It would appear that considerable calculation could be avoided and at the same time accurate results could be obtained, if the chart were arranged to give the relative air density instead of the correction factor. The correction factors, for a wide range of relative air densities for four standard sizes of sphere gaps, are given in the 1922 A. I. A. E. Rules, Table 205, page 23: curves may readily be plotted from the data in the table. In order to find a correction factor, the procedure would be (1) to read the relative air density from the chart at the given temperature and pressure and (2) to read the corresponding correction factor from the curve for the proper sphere gap. Neither of the two steps presents any real difficulties, as the chart is easy to use, while the spread of the correction factor curves is small for the data given in the table; particularly in the range of relative air densities from 0.80 to 1.10. A tabulation of data used in a chart covering a range of relative air densities from 0.80 to 1.10 is given herewith: in addition to these data, it is necessary to obtain the logarithms of several values of the relative air density from a table of logarithms. The tabulation and the chart are based on the formula: