The Wimshurst Machine: Separating and Storing Charges
The Wimshurst machine, like the van de Graaff generator, is capable of throwing long sparks as much as several centimeters between two small conductive spheres. The Wimshurst machine is usually also tied to a Leyden jar, which presents a good opportunity to explore capacitance and charge storage.
What You Need
- Wimshurst machine
- CAUTION: Electrical circuitry including pacemakers, hearing aids, cell phones, flash drives, electronic car door locks, and computers may be damaged by the sparks generated in this experiment. In addition, follow all safety instructions provided by the manufacturer of this device.
- Set the Wimshurst apparatus on a table.
- Darken the room.
- Make sure all electrical jumpers are in place for your particular setup. Check with the manufacturer's instructions to make sure the apparatus is set up properly with correct electrical paths to the Leyden jars and discharge spheres. You can do this both with the Leyden jars connected or not connected to your circuit.
- Separate the two discharge spheres by more than 8 cm.
- Turn the handle for five seconds or so, as shown in Figure 98-1.
- Holding only the insulated wooden handles for the discharge spheres, slowly bring them together.
- Note the distance between the discharge spheres when the first discharge occurs.
- Set the discharge spheres at roughly the same slightly closer and slightly further distances.
- If the Leyden jars were in your circuit, repeat to see what happens when they are not connected.
- Touch the two spheres together for a few seconds when finished to make sure no residual charges are on the electrodes.
With the discharged spheres separated by a large distance, nothing should happen. As the spheres approach to within a few centimeters separating them, a lightning bolt will jump across the gap, as shown in Figure 98-2.
Why It Works
The Wimshurst machine is constructed from two parallel plates made from insulating material such as Lucite or glass. The plates are arranged to be turned by hand in opposite directions. Narrow metal strips are mounted on the plates and oriented along the radius. Charges are transferred by metal brushes that sweep across the metal strips as the plates rotate. In contrast to the van de Graaff generator, the Wimshurst machine separates charge by the principle of induction rather than friction. Positive and negative charges accumulate, and they can either charge a Leyden jar or discharge across a gap.
Other Things to Try
Charges separated by the Wimshurst machine can be determined using an electroscope, as described in the previous experiment.
This experiment shows how, through the movement of two insulating plates near each other, static electric charges can build up. The separated charges can be stored or discharged across a small nonconductive gap.
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