The first article I ever read on the most excellent Evil Mad Scientist Laboratories site was their one on how to build a homopolar motor.
Go there. Check it out. Build one. It's ridiculously easy, and it works remarkably well. And, unlike some other unlikely motor designs, it's unlikely to rip skin off your thumb and then become red hot.
Unsurprisingly, homopolar motors have become something of a GooTube phenomenon, and there've been some innovations.
The Evil Mad Scientist version of the motor has four parts; one battery, one screw, one magnet, one bit of wire.
This can be reduced to three parts by making the magnet static and turning the wire into the rotor:
The "roller" variant.
An elegant spiral version.
The screw type, turned upside down!
Balance this one properly and it could be quite impressive. Using hard disk components is definitely a good way to start.
This one's quite imposing, though the timidity of its operator suggests it's not very well balanced, either.
OK, now this is just showing off.
Before this newfangled fad for homopolarity, there was another "world's simplest motor" that also needed only three components, if you chose those components carefully (it's mentioned on the Evil Mad Scientist homopolar page). Kids who want to get a solid C on their science project can buy a kit to build one.
The old "World's Simplest" motor is considerably more complex than the homopolar motor, but it's also much closer in design to a standard commutated DC motor.
Here's a home-made one in action:
And here's how to make one:
19 July 2007 at 4:23 pm
Yeegh. The second-last video's creator has a rather nasty looking scab or something on his left hand... bit of experimenting with those before he got one that worked, eh? Also... what kind of battery is that? I don't think I've seen one like that before.
19 July 2007 at 6:01 pm
That place (Evil Mad Scientist Labs) looks interesting. They even do my style of cooking
(in essence plugging a sausage into a socket...)
I used to do that as a kid, and with 230V (European voltage) it's much faster than their 1-2 minutes. More like 15-20 seconds per hotdog.
20 July 2007 at 6:32 pm
Ah, evilspoons, that battery is a 4.5V 'pocket light battery. aka 3LR12 (as it says on it...) They used to be fairly popular as low-cost flashlights - you just clip a plastic thingy on top that contains a lightbulb that's neatly placed just between the terminals.
It's a battery, rather than a cell - it contains three batteries of a size rarely seen anywhere else, slightly thicker than AA and much longer.
21 July 2007 at 4:42 am
It surprises me how quickly some these spin up; the length of wire that is actually perpendicular to the magnetic field is quite short in most of these designs. I guess the slight curve of the magnetic field around the battery gives enough of a perpendicular component (relative to the wire around the battery) to add up and make some torque.
21 July 2007 at 5:48 am
The torque from the standard cylindrical designs is very small, but the mass of the rotor isn't large, and there's very little friction, too.
If you can hold the "brush" wire in place properly, the standard version with one 1.5V cell and a dangling screw only takes several seconds before it's spinning fast enough that the inevitable slight mis-centreing of the magnet causes it to zip off to parts unknown :-).