Title: Marx Generator demo
Activity/Demo overview:
This physics demonstration involves charging a large capacitor bank and then instantaneously discharging the built up charge, resulting in a mock lightning bolt being produced by the discharge point on the device. While this demonstration could be used to explain physics concepts such as capacitors or RC (resistor-capacitor) circuits, it was used here to discuss common myths about lightning.
Activity/Demo Instructor Notes:
Main Overview of Instructions:
Before the show
- Ensure that tubing has been filled with copper sulfate solution of proper concentration (this acts as a liquid resistor for the circuit)
- We want roughly 1 MOhms of resistance
- Get Steve to help out with this part
- Make sure that there are no large bubbles or breaks in the liquid resistor
- Large bubbles effectively open the circuit acting as another switch and can negatively affect the discharging of the device
- Make sure everything is plugged in:
- Generator plug
- Control panel plug
- Check that ground wire has been connected
- Check the spark gap wire is roughly halfway between the two metal contacts
- Steve or you may adjust this if the device is not discharging properly
- Make sure that discharge wand is near generator or easily available
- Ensure that discharge point has been mounted/secured
- Either black ceramic resistor or wooden rod secured in clip
- Test discharge/fire and discharge remaining charge with wand
- Prepare wooden rods
- Soak in salt water overnight and dry them (alternative is to boil them in salt water and let them dry)
- Insert nails into both ends
- Place safety glasses for audience
During the show/demo
- NOTE: this is personally the most temperamental physics demo that I’ve done…it is prone to either automatically discharging without user input or not discharging at all…it is highly recommended to do a few tests before the shows to get a little used to it, but you may have to improv all the same when it fails
-
- Turn switch on generator as you walk onto the stage
- Talk to audience and brief them on the loud noise and bright flash the device will make
- Turn switch on main control panel
- Turn dial to max and lift switch cover
- NOTE: you should have done the test firing before the show to have an idea of the charging time on the device…if device is left to charge for too long, the odds of an auto-discharge increase sharply
- Begin countdown for audience
- Press switch to discharge the device
- IMMEDIATELY after discharging, turn the dial down to the minimum to prevent another charging phase
- If loading wood, the demo is over, or if you need to approach the device, grab the discharge wand and carefully rake it over the contacts for the capacitors, ensuring that the balls of the wand touch BOTH contact points of the capacitors to remove any excess charge
- I’d personally also recommend touching the wand across the main discharge gap to remove the small excess charge that builds there…especially once the demo is over (don’t want anyone coming down after the show to get shocked – can also do this with your hand after the show since MOST of the excess charge should be gone)
Main Safety Concerns:
-
- Large discharge of electricity in a small amount of time (~500,000 V)
- High potential/likelihood for discharging automatically without user input
-
- This partly depends on how long the device is left to charge (ie if left to charge for too long then the odds increase)
- However, auto-discharge can sometimes occur regardless of charging time
- Also depends on the concentration of the copper sulfate solution
- If the resistance is lower than expected, it can potentially happen faster
- This can also depend on the placement of the wire in the spark gap that starts the electric discharge cascade
- Potential for accidental electrocution if the device is not discharged with the wand following its use
- (If wood is being split) Potential to send chunks/splinters of wood towards audience
- EMP generated by device should be small but possibly something to be mindful of