Mineral Oil Barometer
By Paul R. Rannie, Popular Science, 1937.
Here is a simple type of barometer that is less expensive and in some ways easier to construct than a mercury barometer, yet accurate enough for amateur weather forecasting and general experimental use. It has a scale three or four times as large as that on a mercury barometer.
A glass tube about 1/8 or 3/16 in. in inside diameter and about 24 in. long is required, as well as a 2-oz. bottle with cork.
Clean and dry the bottle. Make a hole in the cork for the glass tube. Soak the cork in melted paraffin. Pour ordinary mineral oil (such as is sold under various names for medicinal use) into the bottle until there is about 1 ½ oz. of air (by volume, of course) above the oil if the glass tube is 1/8 in. in inside diameter. If the tube is larger, the volume of air should be increased.
Clean the tube and dry it by drawing (not blowing) air through it. Push it through the hole in the cork and insert the cork in the bottle. The tube should extend almost to the bottom of the bottle. Run melted paraffin over the cork.
This is now really an “air thermometer.” Obtain an ordinary household thermometer and place both it and the air thermometer in a pan or glass of water. Cool them to 50 deg. F. by adding ice to the water. At 50 deg., the top of the oil column should be from 3 to 5 in. above the cork. If too low, blow a few bubbles at a time through the tube and into the bottle. Hold the temperature steady for five or ten minutes so the reading will become constant, and mark it with a string.
Warm the water to 60 deg. And continue by 10-deg. Intervals up to 100, marking the tube with string at each step. The oil should rise about 12 in. between 50 and 100 deg. If the oil rises too fast, there is too much air in the bottle, so the cork should be removed and more oil added.
Now complete the barometer as shown. Only the movable temperature scale requires special comment. It is first enameled white, then the gloss is dulled by rubbing it with an eraser so the scale may be inked more easily. The scale may be started at any convenient point, using the distances between the strings as a guide. It is accurate enough to divide each 10-deg. division equally into ten parts.
To make the altitude scale, which goes on the left side of the movable scale, measure the distance between 70 and 82 on the thermometer scale you have just made and let it equal 1 in. of barometric pressure. (The writer determined this proportion by experiment.) Place the 30-in. mark at about the center of the adjustable scale, and lay out the pressure scale below and above it to represent 29 in. and 31 in. Hang the completed barometer indoors and away from drafts or direct heat.
To adjust the instrument, slide the adjustable temperature scale until it reads the same as the ordinary household thermometer. Then you must obtain what is known as the “reduced-to-sea-level” barometric pressure so you can adjust the pointer to its correct permanent place on the pressure scale. You can get this reading from a weather-bureau office, if there is one near you, or by listening to radio weather reports. If you cannot get an exact reading, wait until the weather is about average, that is, neither extremely cold or stormy and set the pointer at 30. You can get the exact reading later on when convenient.
After the pointer on the pressure scale has been adjusted, the barometer is ready to use. Simply slide the adjustable scale so that it reads the same as the regular thermometer, and the pointer will be opposite the barometric pressure. Always note the previous reading before making a new adjustment, because the direction and amount of change is most important. There may be a gradual shift in the pressure readings for a short time after the instrument is constructed, so check the instrument for a few weeks and make any necessary corrections.