fuels usually give a coating which is too coarse grained and heavy. In smoking the drum a long rod is used as a rotating axis. The drum is rotated rapidly in the flame so as to prevent overheating and oxidizing of the foil. The carbon should not be so thick as to obscure the metallic appearance of the aluminum foil.
(e) Calibrate the barograph for pressure. The instrument is placed in a bell jar and the air evacuated. The pressure is kept constant at a number of pressures so that as the drum turns a step, record is made on the smoked foil. Pressure recording starts at about 500 millibars so the first level in the calibration should be at that value. At each level the pressure should be kept constant for three to five minutes in order to obtain a measurable line. Great care and considerable practise are required to control the valves of the vacuum system so that the pressure does not change noticeably during each step.
The pressure steps at which the barograph is calibrated may be either at regular pressure intervals or at the pressure values corresponding to regular height intervals according to the standard atmosphere figures. The recommended steps are listed below. If the balloon is not expected to go to the higher altitudes, the calibration may be stopped at correspondingly higher pressures.
Pressures Standard Atmosphere Heights
500mb 466 mb corresponding to 20,000ft.
400mb 300 mb " 30,000ft.
300mb 188.5 mb " 40,000ft.
200mb 117 mb " 50,000ft.
150mb 72.8 mb " 60,000ft.
100mb 45.3 mb " 70,000ft.
50mb 28.2 mb " 80,000ft.
10mb 17.5 mb " 90,000ft.
10.9 mb " 100,000ft.
The temperature calibration may be made by recording two widely spaced temperatures, such as room temperature and the temperature of dry ice $ (-78^{\circ} \mathrm{C}) $ . This calibration will be approximately a straight line and, therefore, two points are sufficient to plot the curve.