Wimshurst Machine K-12 Information for Lesson Plans & Science Fair Projects

Suppose that the conditions are as in the figure that is the segment A1 is positive and the segment B1 negative. Now, as A1 moves to the left and B1 to the right, their potentials will rise on account of the work done in separating them against attraction. When A1 comes opposite the segment B2 of the B plate, which is now in contact with the brush Y, it will be at a high positive potential, and will therefore cause a displacement of electricity along the the conductor between Y and Y1 bringing a large negative charge on B1 and sending a positive charge to the segment touching. As A1 moves on, it passes near the brush Z and is partially discharged into the external circuit. It then passes on until, on touching the brush X it is put in connection with X, and has a new charge, this time negative, driven into it by induction from B2. Positive electricity, then, being carried by the conducting patches from right to left on the upper half of the A plate, and negative from left to right on its lower half.

The Wimshurst machine is a historical electrostatic machine for generating high voltages, and was developed between 1880 and 1883 by British inventor James Wimshurst (1832 – 1903). It is an electrical generator with a distinctive appearance, having two large contra-rotating discs mounted in a vertical plane, two crossed bars across them, and a spark gap formed by two metal spheres.

Description

These machines belong to a class of generators called influence machines, which create electric charges through electrostatic induction, or influence. Earlier machines in this class were developed by Wilhelm Holtz (1865 and 1867), August Toepler (1865), and J. Robert Voss (1880). They were more efficient than the earlier machines that worked by friction. The older influence machines exhibited an unpredictable tendency to switch their polarity. The Wimshurst did not have this defect.

The basic operating principle of a Wimshurst machine is that as the two insulated counter-rotating disks porting metal sectors pass the “crossed metal neutralizers bars with brushes, amplifying charge imbalances are induced. The disk generated charges are then partially transferred to the two sharp metal combing points mounted on horseshoe-shaped collecting conductors that are placed small distances from the surfaces of each disk at the sides of the machine. The collectors are mounted on insulating supports, and connected to the output terminals. This positive feedback effectively increases the accumulating charges exponentially until the ionization of the air and sparking potential no longer limit the charge concentration.

The machine is self-starting, meaning external electrical power is not required to create the initial charge. Such machines quickly amplify the ever-present minute charge imbalances in the rotating disks. It does require mechanical power however, to turn the disks against the electric field, and it is this energy that the machine converts into electric power. The output of the Wimshurst machine is essentially a constant current, that is proportional to the area covered by the porting metal sectors and the rotation speed. The acccumulated spark energy can be increased by adding a pair of Leyden jars, an early type of capacitor suitable for high voltages, with the inner Jars’ plates independently connected to each of the output terminals and the outer Jars’ plates interconnected. A typical Wimshurst machine can produce sparks that are about a third of the disk's diameter in length, and several tens of microamperes of current.

Description

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