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The National MagLab is funded by the National Science Foundation and the State of Florida.

Kelvin Water Dropper

The legendary Lord Kelvin made electricity from water with his water dropper.

William Thomson, better known as Lord Kelvin, invented an ingenious electrostatic generator known as a Kelvin water dropper.

Water does not exhibit an overall electric charge under normal circumstances, but contains many ions from salts dissolved in the liquid. Some of the ions are positively charged (cations) and others are negatively charged (anions), so that they typically balance each other out.

If a charged object is placed close to water, however, the ions in the water will separate into groups. Ions with an opposite charge will draw closer to the object. The object will repel ions with the same charge. This is the premise that the Kelvin water dropper is built upon.


  • See how this device is set up, with water from a single source directed into two separate metal buckets. Above each of the buckets are metal rings. The rings are wired to the bucket on the opposing side.
  • Each bucket also is connected to a ball-tipped Discharge Rod positioned so that it is only a short distance away from the rod on the other bucket.
  • The flow of water is adjusted so that it falls in droplets, rather than in a continuous stream.
  • Note how a slightly charged negative droplet of water falls through the left side of the device. Because the electrons in the metal ring are repelled by the negative charge, they move away, down the attached wire, so that the ring is left with a slight positive charge.
  • The now positively charged ring is more likely to attract additional negatively charged water droplets, and as these droplets pass through the ring they render it even more positive, amplifying the effect.
  • The negatively charged particles that fall into the bucket below the ring transfer their charge to the bucket, providing it with a negative charge.
  • At the same time, the metal ring on the right side of the instrument develops a negative charge. This results from positively charged water droplets and the increasing proportion of positive ions in the water from the ring on the left.
  • As more and more flow through each ring, the opposing charges increase in the rings and in the buckets below, creating a feedback system characteristic of the Kelvin water dropper.
  • Once the charge separation begins, the feedback produces a large potential difference relatively quickly. The voltage soon builds up to such an extent that the water dropper discharges, generating a spark between the conducting rods.
  • Move the slider to change the separation between the rods, increasing or decreasing the rate of discharge. Then the process of charge separation begins anew and the cycle of electrostatic generation continues.