When a bubble, or spirit, level is level, what does that mean relative to Earth?
Pat French
Longdon-upon-Tern, Shropshire, UK
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There is a well-known story of a gentleman who held that Earth was flat. He took a spirit level on a long-haul flight and afterwards announced that, other than during take-off and landing, the aircraft had flown level over the flat Earth.
When the bubble in a spirit level lies centrally in the fluid, it means that the gravitational pull on that fluid is uniform across the container and either side of the bubble. At this point, the position of least resistance for the bubble in the fluid lies at the centre of the tube. This effect is often facilitated and stabilised by a barely visible bend in the cylinder.
Gravity works towards Earth’s centre. A line that extends from any point on the planet’s surface to its centre of gravity (at the core) is what we refer to as “vertical”.
When level, provided that the tube containing the fluid plus bubble is mounted accurately in the spirit level, then the instrument base will be at 90 degrees to a vertical line of gravitational pull regardless of latitude – be it the polar circles, tropics or equator. The local topography – the humps and bumps on Earth’s surface – make no practical difference. We call this orientation “horizontal”.
An accurately calibrated spirit level essentially tells you the direction in which gravity is acting at that point
Mel Earp
Macclesfield, Cheshire, UK
An accurately calibrated spirit level essentially tells you the direction of gravity at that point. To test a surface, you need to take two measurements at right angles. For example, one in a north-south direction and one east-west. Because Earth isn’t an exact sphere – it bulges out slightly at the equator because of its rotation – the flat surface that you have just measured isn’t exactly tangential to Earth’s curvature at that point, unless you are at the equator or one of the poles.
The World Geodetic System of 1984 (WGS84) defines a standard Earth coordinate system and Earth shape in the form of an ellipsoid. It also defines a gravitational model. Your spirit level will experience a gravitational field very similar to this, but not identical. There will be deviations due to local topography, such as hills and valleys, and also variations in the density of Earth deep beneath your feet. These will cause local deviations in the gravitational direction with respect to the WGS84 model. But they are tiny – of the order of one-hundredth of a degree – even in extreme circumstances such as in the Himalayas. This is far less than any error there might be in the manufacture of the spirit level, and indeed one’s ability to read it, and can pretty much be ignored. So you can be sure that the objects you place on your newly fitted shelf will be grateful for gravity not acting sideways.
Hillary Shaw
Newport, Shropshire, UK
Earth’s centre of gravity is also its geometric centre, and so both ends of the spirit level are the same distance from that centre when the device is level. It follows that the bubble in the middle is very slightly nearer. Assuming a 50-centimetre-long level that is totally rigid (and very precise), the bubble will be around 0.004 micrometres nearer Earth’s centre. That is about the same as a stack of 10 plutonium atoms.
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