Kelvin generator

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The Kelvin generator or Kelvin water drop generator is a Influenzmaschine that the energy to establish the electrical voltage from the kinetic energy of wins of falling water drops which in electrostatic field are decelerated. The device was described in 1867 by William Thomson , who became Lord Kelvin of Largs in 1892.


The electrostatic influence caused by the electrical charges of the upper, smaller cylinder causes that at the ends of the still closed water column the charges of different names are attracted to the charges of the cylinders, which are then transported down with the falling drops . These charged drops fall from the upper, smaller, unequally charged cylinders into the lower, larger, identically charged cylinders, which contain cone-shaped funnels inside , the drops being slowed down by the electrostatic field. The drops give off their charge in the funnels of the lower cylinders because the mutually repelling charges are distributed on the outside of the metallic conductive cylinder.

The side of the Kelvin generator on which the positive and negative voltage builds up depends either on chance or on an electrostatic charge that has already been given up. As long as the Kelvin generator is not completely discharged, it increases its voltage through the positive feedback of the cross-connected upper and lower cylinders until a spark occurs, or until the water drops are repelled by the electrostatic field from the lower cylinders and into spray in all directions.

Smaller devices, with lower cylinders of, for example, 60 mm in height and diameter, as shown in the pictures, reach a voltage of around 3000  volts , which corresponds to a spark gap of around 3 mm. To see the discharges better you can before the spark gap in series connection nor a 220-volt neon lamp installed.

Energy balance

The electrical energy of the Kelvin generator comes from the potential energy of the water dripping from above. Falling is slowed down by electrostatic repulsion from below and by electrostatic attraction from above, so that part of the kinetic energy when falling is converted into electrical field energy. In the case of high charges, the proportion can become so high that the droplets cannot reach the funnels because they have already been slowed down to zero falling speed and now move to the side in the inhomogeneous field.

The mechanical energy of the water droplets can be partially “recovered”, for example by using a flutter-leaf electroscope .

The discontinuous device

The discontinuous device (in the right picture in the picture row) is even simpler , but it has the disadvantage that you have to empty your cups from time to time. On glass as insulator should dispense with electrostatic machines in general because it wins by forming a thin water skin a noticeable surface conductivity. Water-repellent plastics are much better suited as insulators .

Other liquids

The Kelvin generator can also be operated with distilled water (deionized water). The autoprotolysis of the water ensures the necessary (low) conductivity. The conductive mercury would be particularly well suited due to its high density , but the toxic vapors formed pose a problem. The liquids used must have a certain conductivity in order to ensure equipotential bonding between the two drip points and also within the funnel.

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