Put slightly moist earth into the available vessel. Apply V ≈ 30 volts direct voltage on the electrodes put down in parallel on top of the earth in a distance of 10 cm, thus a near homogeneous electrical field will be formed in the near-surface layer of the wet soil. Extend the LED electrodes in a distance of 2 cm. The LED stuck into the sand would illuminate subject to the size and polarity of the voltage between the bent out legs.

Opening direction polarity:
V ≈ (30 volts)/(10 cm) ∙2 cm ≈ 6 volts
The LED lights up brightly.

When it is turned away a bit:
V ≈ (30 volts)/(10 cm) ∙1.4 cm ≈ 4,2 volts
The LED still lights up sufficiently.

When it is turned away a bit further:
V ≈ (30 volts)/(10 cm) ∙0,7 cm ≈ 2,1 volts
The LED lights up just a little.

If the LED is stuck in the earth so that the straight line connecting its legs would lie in parallel with the external electrode, the legs will get to an equipotential line: the LED would not light up.
If turned any further in the same direction, the LED will still not illuminate, since it gets a blocking voltage. V < 0
The legs of the LED turned away in 180o compared to the initial position will receive a potential difference of V ≈  -6 volts, therefore  the LED would not light up.

In an inhomogeneous electrical field the LED would illuminate more intensively or less strongly subject to the direction of sticking and the local voltage level in between the legs (spot electrodes, red LEDs).

Implements required for the experiment

– 30 volts direct current voltage supply
– A minimum 18-20 cm wide vessel
– Slightly wet earth
– Two metal pieces with parallel sides as electrodes (about 1 cm × 3 cm × 10 cm)
– 1 cm diameter LED (red or green, if possible)
– Wires
– Voltmeter
– Ruler