A garden gnome from Germany has visited a Canadian lab located two kilometres underground as part of a project to show how the effects of gravity vary around the globe.

"It's a fun demonstration," Chris Jillings, a research scientist at Snolab in Sudbury, Ont., said Friday after the facility hosted Kern the gnome and conducted measurements for the educational and public science project known as the Gnome Experiment.

The project has already taken Kern to destinations including London, Mumbai, Tokyo, Lima and San Francisco. At each one, he was placed on the same precise scale to detect small differences in gravitational effects, as weight is a measurement of the pull of the Earth's gravity on an object. The scale, which travels with the gnome, is made by Balingen, Germany-based Kern & Sohns Precision Scales, which is organizing the experiment.

Mass versus weight

Grams are usually a unit of mass, not weight. While weight changes with variations in gravity, mass is a measure of the quantity of matter, which does not change with variations in gravity. However, most scales on Earth are technically able to measure only weight. They are calibrated to convert the measurement to mass in grams, and the calibration varies depending on the location. However, in this case the scale was only calibrated once – in Balingen, Germany. That is why the gnome weighed at a different number of grams at each location even though his mass didn't actually change.

During his Canadian visit, Kern was weighed both on the surface and two kilometres underground. Snolab's underground facilities are known as the deepest underground multi-purpose lab in the world.

"When we measured the gnome underground, it actually came out about 0.11 grams lighter than on the surface," Jillings said.

According to the scale, Kern weighed 307.73 grams on the surface and 307.62 grams underground.

"The reason it weighs a little bit less underground is that … there's less rock underneath you pulling you in," Jillings said.

When the gnome is deep underground, the gravitational effects of the two kilometres of rock above it are pulling it toward the surface and away from the centre of the Earth. That cancels out some of the gravitational pull of the rock beneath it.

"It's a small effect," Jillings said, "but with a good balance, you can certainly see that the weight of things is slightly different."

A map of Kern's journey so far, posted on the Gnome Experiment website, shows measurements of his weight vary from location to location.

Gnome gains weight at South Pole

Jillings said two other main effects can lead to small differences in the measurements. One is the rotation of the Earth, which applies a force that can reduce the effects of gravity near the equator.

Travelling gnome prank

The experiment is based on the popular "travelling gnome prank" in which a garden gnome is stolen from outside someone's home. It is then photographed in different parts of the world and the photos are sent to the gnome's owner.

"You could imagine a tennis ball at the end of a string you're swinging around," Jilling said. Part of the force of the string on the tennis ball — analogous to gravity — is used to keep it moving in a circle instead of pulling it toward the centre.

A gnome standing on the North or South Pole would be rotating on a single point rather than swinging in a wide circle, and would therefore feel the full effects of the Earth's gravity.

The second effect that can change an object's weight with location is that the Earth is slightly flattened at the poles. That means its radius at the equator is about 20 kilometres bigger than at the poles, Jillings said. Objects at the equator are therefore slightly farther away from the centre of the Earth, so the gravitational force is not as strong.

Kern was found to weigh the most at the South Pole — 309.82 grams — his last stop before Snolab.

Variations in gravitational effects at different locations on Earth are quite small, Jillings said, changing the weight of an object no more than half a per cent in total as it moves between the equator and the poles. Because of that, participants in the experiment were asked to ensure no other factors altered the weight of the gnome.

"The rules actually were quite careful," Kern said. "We had to handle gnome with gloves and so on, to make sure it would stay clean and so not get chipped and what-not."

On Friday, the gnome was en route to its next destination, the CERN laboratory at the French-Swiss border, home of the Large Hadron Collider.