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even when made by different observers, rarely differ by more than 09.03 F.

After all the loaded holders have been read at the first test point, the bath is brought to the next test temperature, 100° F., and the clinical thermometers are again dipped and read in the manner described. This procedure is repeated for the other test points, 104° and 108° F. The holders are then mounted two at a time in the whirling machine and the mercury is made to retreat below 95°, and the entire series of observations at the four test points is repeated.

The record sheet previously referred to on page 283 is the record for one holder. The corrections at each test point as determined by the two independent tests are entered in columns 6, 10, 14, and 18, and the means of the two tests to the nearest 00.1 are entered in columns 7, 11, 15, and 19. In the test record sheet shown thermometers Nos. 5 and 17 were rejected because the indices were too difficult to throw back, No. 8 failed to hold its index ("retreater"), No. 11 because the error exceeded the allowable error (00.3), and Nos. 14 and 22 because they failed to repeat their readings within 0°.15 F. in the two tests. The remaining thermometers fulfilled all the test requirements and were then given B. S. serial numbers, which are entered in column 3.

B. S. IDENTIFICATION MARKS.

The serial numbers just referred to, preceded by the letters B. S., are then etched on the thermometers. To accomplish this the upper parts of the stems are thoroughly cleaned and dried, and then covered with a thin coating of wax by immersion in a pot of molten wax. The extreme ends are further protected by a second dipping before engraying. The wax pot used in this work has proved so convenient that a brief description will be given. It consists of a double-wall vessel wound with two heating coils connected to a lighting circuit by an ordinary fuse-plug connector. To melt the cold wax quickly the two coils are thrown in parallel, and when the wax is all melted it is kept at a constant temperature by the two coils in series.

The letters B. S. and the serial numbers are engraved on the back of the thermometers by means of an engraving machine made by Eaton and Glover. This machine, though somewhat large for this character of work, is nevertheless quite convenient. The etching is done by immersing the end of the thermometer for a short time in a bath of hydrofluoric acid.

CERTIFICATE.

The corrections to the thermometers are then transferred from the test record to the certificates, a facsimile of which is shown in fig. 11.

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