Poles.A perfect sphere of spherically uniform density (density varies solely with distance from centre) would produce a gravitational field of uniform magnitude at all points on its surface, always pointing directly in the direction of the sphere's centre. Furthermore, the net force exerted on an object due to the Earth, called 'effective gravity' or 'apparent gravity', varies due to the presence of other factors, such as inertial response to the Earth's rotation. However, the Earth deviates slightly from this ideal, & there are consequently slight deviations in both the magnitude & direction of gravity across its surface. A scale or plumb bob measures only this effective gravity. The major reason for the difference in gravity at different latitudes is that the Earth's equatorial bulge (itself also caused by inertia) causes objects at the Equator to be farther from the planet's centre than objects at the poles. As the force due to gravitational attraction between two bodies (the Earth & the object being weighed) varies inversely with the square of the distance between them, an object at the Equator experiences a weaker gravitational pull than an object at the poles.
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