When a barometer is inclined so that the mercury is near the top of the tube, a slight lengthwise movement will cause it to flow to the top, striking it with an audible “click.” There is a tradition that the character of the vacuum can be determined by the character of the sound; but inasmuch as trained experts are sometimes deceived, the value of the click as a test is uncertain; and inasmuch as such a practise is likely to break the tube, the negative value is pretty certain.
Barometer Scales and Standards.—For many years barometric pressure was expressed in the linear units of the country. The adoption of the metric system in several states of Europe changed the use of local units to metric units. The metric system has been authorized to be used in the United States, but the use has not been made compulsory. It is employed in laboratories and in certain scientific work, but not in the manufacture of precision machinery unless definitely ordered. It is not used for commercial purposes in English-speaking countries. In the latter, barometric pressure is expressed in inches. Metric scale barometers are furnished on order by the makers.
So far as choice between the two scales is concerned there is not much difference. Each is intelligible in the locality where it is used. So far as the keeping of records is concerned there is neither gain nor loss; each requires four figures and a decimal point.
Physicists who use the metric system of measurements find it convenient to use the dyne—a force that will impart to one gram an acceleration in velocity of one centimeter per second—as the unit of pressure. The pressure base proposed for barometric measurements is 1,000,000 dynes. This value is not sea level pressure, but the average pressure at a height of 106 meters (348 feet) above sea level. The unit is the kilobar, or 1000 millibars. The conventional atmosphere of 29.92 inches is 1013.2 millibars.
To the great majority of observers any barometer scale is more or less empiric. By long training and habit one gradually acquires a mental value of the figures which express pressure and these become visual proportions that can be compared in the mind. It is difficult to change the results of this education; it likewise requires time. So far as expression of barometric terms of pressure are concerned, there is not the slightest gain
