Pressure in a gas pipe may be measured by a simple instrument called the pressure gauge:  The gauge consists of a bent glass tube containing mercury, and so made that one end can be fitted to a gas jet (Fig. 52).  When the gas cock is closed, the mercury stands at the same level in both arms, but when the cock is opened, the gas whose pressure is being measured forces the mercury up the opposite arm.  If the pressure of the gas is small, the mercury changes its level but very little.  It is clear that the height of a column of mercury is a measure of the gas pressure.  Now it is known that one cubic inch of mercury weighs about half a pound.  Hence a column of mercury one inch high indicates a pressure of about one half pound to the square inch; a column two inches high indicates a pressure of about one pound to the square inch, and so on.

This is a very convenient way to measure the pressure of the illuminating gas in our homes and offices.  The gauge is attached to the gas burner and the pressure is read by means of a scale attached to the gauge. (See Laboratory Manual.)

In order to have satisfactory illumination, the pressure must be strong enough to give a steady, broad flame.  If the flame from any gas jet is flickering and weak, it is usually an indication of insufficient pressure and the gas company should investigate conditions and see to it that the consumer receives his proper value.

87.  The Gas Meter.  Most householders are deeply interested in the actual amount of gas which they consume (gas is charged for according to the number of cubic feet used), and therefore they should be able to read the gas meter which indicates their consumption of gas.  Such gas meters are furnished by the companies, and can be read easily.

[Illustration:  FIG. 53.—­The gas meter indicates the number of cubic feet of gas consumed.]

The instrument itself is somewhat complex.  It will suffice to say that within the meter box are thin disks which are moved by the stream of gas that passes them.  This movement of the disks is recorded by clockwork devices on a dial face.  In this way, the number of cubic feet of gas which pass through the meter is automatically registered.

89.  The Relation between Pressure and Volume.  It was long known that as the pressure of a gas increases, that is, as it becomes compressed, its volume decreases, but Robert Boyle was the first to determine the exact relation between the volume and the pressure of a gas.  He did this in a very simple manner.

Pour mercury into a U-shaped tube until the level of the mercury in the closed end of the tube is the same as the level in the open end.  The air in the long arm is pressing upon the mercury in that arm, and is tending to force it up the short arm.  The air in the short closed arm is pressing down upon the mercury in that arm and tending to send it up the long arm.  Since the mercury is at the same level in the two arms, the pressure in the long arm must be equal to the pressure in the short arm.  But the long arm is open, and the pressure in that arm is the pressure of the atmosphere.  Therefore the pressure in the short arm must be one atmosphere.  Measure the distance bc between the top of the mercury and the closed end of the tube.