We’ve seen it in old movies: the mad scientist in his lab full of weird equipment, oddly shaped glassware and strange machines. There’s always a Jacob’s Ladder.
There’s an electric arc that starts at the bottom of the V-shape, travels to the top, then vanishes and starts from the bottom again.
Does such a device actually have a use as a piece of lab equipment, or as a component of a larger machine?
It’s a pretty good way of stress-testing an HV power supply with minimal equipment. If the supply can’t generate high voltages, it won’t work at all. And if the current limiting isn’t working, it’ll handily blow up your supply, or at least trip a breaker.
Aside from that, I’m not aware of any actual use. Not that that’s stopped me from building them. I think the rods might still be attached to my neon sign transformer, in fact.
That said, there are some mildly analogous situations where an arc isn’t exactly desired, but is an inevitable result of a desired operation. For instance, high voltage disconnect switches:
Their wiki page doesn’t even list much more than them being fun to look at. I’m kinda surprised as I’ve seen a number of electrical projects/equipment (think: ElectroBoom type stuff) that require some an arc for one reason or another.
I’m guessing a big part of why they were popular in movies, especially older movies is that, as you alluded to, they’re not terribly difficult to build. I have few largish (18kv?) neon sign transformers and it takes all of a few minutes to whip something together. And, like you, I’m pretty sure mine is still sitting in the exact same spot it was when I made it, probably 20 years ago.
I’m not aware of any practical uses either, but the same basic idea (generating an electrical arc, without the “climbing” bit) has a lot of uses. An electrical arc can be used to generate ozone, for example, or can be used as an igniter for all sorts of things (the igniters for the burners and oven in a gas stove, the spark plugs in your car, etc).
Arc suppression is a big thing in the power industry. As the video that @Dr.Strangelove posted, a high voltage switch can arc quite a long distance. This is especially a problem in high voltage DC power transmission. High voltage DC is more efficient than high voltage AC since the power lines are constantly running at peak voltage (AC insulation standoffs, wire spacing, etc. has to be designed for the peak since wave voltage but from a practical matter you only “average” the RMS value for how much voltage and power you can get through it). DC also has much lower capacitive and inductive losses, which is especially an issue if your high voltage line is going underwater at some point.
But AC naturally goes to zero volts twice through the sine wave cycle, and DC doesn’t ever go to zero volts since it is constant DC. This means that if you draw an arc (like in the above video) AC will tend to naturally extinguish itself, where DC will naturally want to draw the arc forever. The arc suppression required for DC switch gear, as well as the more complex issue of transforming the voltage up and down at either end of the line, means that you need to overcome all of these extra costs before you can save money using the better efficiency of DC. This makes high voltage DC impractical for shorter distances.
When you are developing this type of high voltage DC switch gear, you are going to need some way to generate a honking big electrical arc, which is basically all of the components of a typical Jacob’s Ladder except that you don’t angle the conductors slightly away from each other to make the arc rise up the “ladder”.
So it’s “almost” a Jacob’s Ladder. But not quite. But the only difference is the shape and orientation of the metal bits at the business end of the device.
In both cases (Jacob’s ladder or high power switching) you’re also relying on the convection of the heated plasma and air to eventually stretch the arc past the breaking point.
Other approaches include arc suppressing air jets and such which try to “blow out” the arc by stretching and dispersing the plasma more effectively than mere atmospheric convection would do.
As far as high voltage things go, Wikipedia mentions several applications for Marx generators and Cockcroft–Walton generators, including high-energy physics experiments.
I used a Marx Bank in a very high-energy laser many years ago. But what we didn’t want were showy lightning-like discharges. We operated the whole thing inside an oil bath. And we still got unwanted arcing. It scored a carbon path across the plastic parts that I had to spend a lot of time scouring out.
I had a 200 kV Cockroft-Walton accelerator in my lab. We did NOT want to see it act like a Jacob’s ladder. We used it to do 400 keV ion implants with doubly charged ions. As an undergraduate, I operated a 10 MV tandem van de Graff generator. It was gigantic. It discharged frequently, since we were always trying to get it to its maximum potential. What a pain. I think the grad students were in a bind since all the easy experiments had already been done, which usually meant they wanted me to try to get the machine to do something it couldn’t do. If they had been content to run at 9 MV, life would have been so much easier.
I don’t know of too many clubs where multiple people own jacobs’ ladders and one guy ran (runs?) a 10MV van de Graff.
But we can throw some bitchin’ parties!
One practical use to mad scientist labs that I can think of is as an operating voltage indicator for the high voltage supply, monsters for the jolting of. The usual LED indicator is a bit impractical here.
Own and have built. That part’s important, too.
Though I’m not sure how much of a secondhand market there is.
Darn, I thought this thread was going to be about the block-and-ribbon toy, and was looking forward to learning about its practical applications.
Well, one link of that, or something similar to it, can be used to make a door hinge that opens either way. And while still a toy, the Rubik’s Magic is a much more elaborate variation of the same idea.
Not Jacob’s Ladder, but spark gap was used for early radio transmissions.
Brian
My first week as an undergrad I saw a bunch of other freshman who brought a transformer, a piece of wood, and some bolts. They took a coathanger apart and broke it into two pieces, which they attached to the board and to the leads from the transformer and had themselves a Jacob’s ladder in their dorm room.
Yeah, but who doesn’t have a Jacob’s ladder in their dorm room? You don’t really need the wood. Neon sign transformers (the usual source for these transformers) typically have bolts on either end that you can screw the bent coathangers right into. Just requires a bit of fiddling to get the narrow spot the right distance.
I did eventually make a Tesla Coil out of the NST, though it never worked very well.
I bought mine used from an actual neon sign shop, though I think the guy ripped me off. Charged me $40 for something he was about to throw away. Still works, though, so I guess I can’t complain.
That was my thought, although I am not an electrical engineer. Is it possible that the higher the voltage (wattage? amperage? somethingelseage?) the higher the arc will climb up the Jacob’s ladder? And by varying the conductivity of the “legs” you can increase/decrease the sensitivity of the device.to come up with an a crude measurement of the power input?
Nobody uses it to climb up a Jacob? Those suckers are tall.
I think it would be very hard to calibrate. The arc wiggles around and doesn’t have a consistent length, and it’s undoubtedly affected by things like air currents and humidity.
The distance along the conductor wouldn’t have any effect. The currents are extremely low (my NST is rated at 50 mA), and the voltages are very high, so there’s almost no voltage drop along the length.