What exactly is "rhetorical effect?

What exactly are Hall Effect Sensors? How exactly are they useful in a Maglev Train Model?

• Okay, I am totally into electrodynamics stuff eversince i got introduced to the topic few months back. I gotta do something for my K12 project and i thought i could do Maglev. As far as I've understood, there are two major methods of building a model maglev - 1. the superconductor method 2. the linear motor method. The superconductor method is way too expensive for a school project so i am left with the Linear Motor method. Now, there are many kids in my school who have done Maglev that work mechanically (by giving an push the train kinda runs), the easiest and the cheapest way to make a train. So mine's gotta be the higher end one. So mine will be an 'automatic' (powered by cells) model, and also intelligent to some extent. So i searched books and the internet and i found this extremely common method to do it - it requires this this device called as 'Hall Effect Sensor". Will be glad if someone tells all about this thing. Thanks a ton in advance!!

• Answer:

  From looking at your Additional Details link, it appears that designer is simply using the Hall Sensor as a switch to know when to change the polarity of the magnetic field levitating the car. Hall Effect Sensors are devices designed to measure the strength and direction of a magnetic field and then convert that measurement into a voltage. They utilize the Hall Effect (wikipedia link in Sources). Basically, you apply a voltage across a wide conductor. As the current flows, the electrons experience the force of the magnetic field which causes them to bend slightly. This makes them bunch up along one side of the conductor, and creates a potential difference between that side and the side opposite it. A Hall Effect Sensor then outputs the voltage across the sides (probably amplified). In this way you can get a good reading of both the strength of the magnetic field the sensor is in (by seeing how big the potential difference is), and which direction it is (by seeing which way the potential difference goes). In the link provided in the Additional Details section of the question, the modeller is building a track of magnets that alternate polarity. That is, as you travel down the track you will see a section of, say North polarity, followed by a section of South polarity. Now, the problem is that as your maglev train moves down the track, the polarity it needs to generate to continue to levitate changes. If it didn't change, it'd get half a section down the track, see inverted polarities and immediately be attracted. The Hall Effect Sensor is used to switch the polarity of the levitating train as it passes down the track. So the modeller has wired some circuitry so that when the sensor sees a North polarity, it makes the train also generate a North polarity, and since like polarities repel, the train levitates. As the train moves down the track, the sensor will see the next section as a South polarity, and switch the train over to a South polarity as well. If tuned appropriately, you could probably get the train to move on its own by having the Hall Sensor offset just slightly from the train. That way as the train moves, it'll be attracted just a little bit to the section of track in front of it, and repelled a bit more by the section of track behind. Edit: Whoops! I almost forgot the wikipedia link. Edit2: On further examination of the link to the project details, I retract my section about using it to keep the train floating. It appears that the Hall Sensor is used only in the drive train section, so it'd work like I describe in the paragraph above the first edit.