William Thomson, often better known as Lord Kelvin, was one of the most influential scientists of the nineteenth century. His interests were diverse; he made significant contributions to mechanics, mathematics, magnetism, electricity, thermodynamics and hydrodynamics. One of many testaments to Thomson’s innovative nature is the device presented in this tutorial, usually referred to as a Kelvin water dropper. Invented in the 1860s, it is an ingenious sort of electrostatic generator.
In this device, water from a single source is directed into two separate Metal Buckets via tubing ending in small nozzles. The flow of water is adjusted precisely so that the water rapidly falls in droplets, rather than in a continuous stream, through Metal Rings located above each bucket. The rings are electrically attached to the bucket on the opposing side; there is no contact between the wires. 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. Periodically a spark will jump across the gap in the conductors when the potential difference between the two buckets can no longer be maintained. (This gap can be adjusted using the Discharge Rod Separation slider; the closer the rods, the more quickly the build-up of charge will be discharged.) The difference in potential that develops is related to the ions (charged particles) present in water.
Water does not exhibit an overall charge under normal circumstances, but contains many ions from salts dissolved in the liquid and the dissociation of water itself. 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. If a charged object is placed close to water, however, the ions in the water will separate into groups. Ions with a charge opposite to that of the object will draw closer to it, ions with the same charge will be repelled. This is the premise that the Kelvin water dropper is built upon.
In our tutorial, 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.
Almost simultaneously, the metal ring on the right side of the instrument develops a negative charge. This results both from the transfer of some of the negative charge accumulating on the surface of the bucket on the left (via the wire that connects it to the ring on the right) and from the increasing proportion of positive ions in the water that flow toward the right because they are repelled by the positive charge at the surface of the ring on the left. Again, as more and more positively charged droplets flow through the right ring, the negative charge on the ring increases, and the positive charge on the bucket below increases. Some of the positive charge on that bucket can then be transferred to the ring on the left, completing the positive feedback system characteristic of the Kelvin water dropper.
Once the process of charge separation is begun, positive 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. A scientist from the time period could have stored the charge in a Leyden jar. Then the process of charge separation begins anew and the cycle of electrostatic generation continues.