Kelvin water dropper

A typical Kelvin water dropper uses two streams of water falling through separate metal rings or cylinders into metal collecting vessels. Each upper ring is electrically connected to the vessel beneath the opposite stream. The two sides are therefore cross-connected rather than simply wired ring-to-container on the same side.

Water contains mobile positive and negative ions, including ions contributed by dissolved salts. A small electric field near one ring influences how charge is distributed in the stream before droplets detach. The drop carries a slight net charge into its collector; the cross-connection then changes the potential of the other ring, influencing the charge of droplets in the other stream.

The arrangement reinforces an initial charge imbalance. For example, a slight negative charge on one collector induces a small positive charge in droplets falling toward it. That charge is transferred to the collector and, through the cross-connection, affects the ring above the opposite stream. Droplets on that side are then driven toward the opposite sign, and their collector further strengthens the original difference. The roles reverse if the initial imbalance has the opposite sign.

As the process continues, charge accumulates on the collectors and the voltage difference grows. The precise behaviour depends on the apparatus, water flow and electrical leakage. When the electric field becomes sufficient for the surrounding air to conduct, a spark can jump between suitably arranged electrodes, reducing the accumulated charge.

Gravity supplies the energy that moves the water downward. In the generator's electric field, the charge carried by the moving droplets is separated and transferred in a way that builds electrical potential energy. The water dropper is therefore not an energy source without input: its electrical output is ultimately supported by the falling water's gravitational energy, along with losses in the system.

The device is a vivid demonstration of induction and feedback: an initial charge that may be very small can be amplified by the arrangement itself. Laboratory demonstrations have reported voltages in the kilovolt range, but output depends on the particular construction and conditions.

The generator needs an initial charge imbalance, which can arise from residual charge or other small asymmetries. If charge leaks away too quickly, the water streams fail to remain well separated, or the electrical connections are not arranged correctly, the feedback may not build to a visible spark. Humidity, surface contamination and the geometry of the apparatus can influence performance.

Although the currents are typically small in classroom models, the voltage can be high enough to produce sparks. Demonstrations should follow the safety procedures specified by the equipment provider and should not be handled as ordinary low-voltage experiments.