Magnet Myths and Misconceptions

[TinselKoala]

7)But in the conventional case, with the field lines strictly parallel to the long axis except near the end poles, the sensor will experience the field lines straight through the plane of the sensor, and thus the sensor's reading will remain constant, and _at the maximum value_  as it is scanned along the magnet's long axis. Right?

No.  Near the ends of the magnet where there is lots of curl, there will be substantial flux, and therefore flux density that is perpendicular to the dipole axis and registered by the senosr.  The sensor should read just as the diagrams I posted indicate.  Near the middle of the magnet where all the flux lines become parallel the perpendicular flux, and therefore flux density is zero.  A plot of the sensor output will be a non-linear, but monotonic line moving along from one end of the magnet to the other.

Think about that again, please. The question referred to a Hall sensor whose plane is perpendicular to the long axis of the magnet. Perhaps you missed that part. If you look at Tinman's drawing above, you can see that in the central region the field lines are roughly parallel to each other and to the long axis of the magnet, and maintain a nearly constant density for some distance. The longer the magnet, the more parallel they are and the more constant the density along the mid portion of the magnet's length. Hence, a perpendicular Hall sensor will experience flux very nearly perpendicular to its plane and also with almost constant density while being scanned along this region. In fact, if the Hall sensor is rotated appropriately at the curly ends, it is possible to maintain a nearly constant reading from fully in contact with the pole, following around the curly corner, scanning along the length of the magnet, then curling around the corner again at the other pole. This action keeps the plane of the Hall sensor perpendicular to the field lines, as drawn in the diagram above, during the entire scan, and the density of the flux intercepted by the Hall sensor can also remain nearly constant throughout the scan including the curling portions, somewhat dependent on the actual geometry of the magnet. A fat "bar" magnet whose width is on the same order as the length will have less concentrated flux at the curling portions than will a long skinny magnet.

[synchro1]

Rotation

"Rotating the magnet or reed switch, normal to their axes, reverses magnetic polarity resulting in two closures per revolution. When these axes are parallel, the switch closes. When the axes are perpendicular, the switch opens. Although the poles reverse, they still induce the opposite poles that close the reed switch".

[TinselKoala]

Here is a really simple experiment. Lets just take a reed switch,LED and a 2.5 volt power source.Hook all in series,and then run the reed switch along the length of the magnet. If the LED stays on,then there is a magnetic field,if it go's out near the center of the dipole,then there is no magnetic field.

How dose a reed switch work-Quote wikipedia- The reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by W. B. Ellwood. It consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass envelope. The contacts may be normally open, closing when a magnetic field is present,

A normal reed switch is not biased, it is just like a bit of iron: Its contacts are attracted by either pole of a magnet. In the central portion of your test, the reed contacts are equally attracted to both poles of the magnet so they don't move. This does _not_ mean that there is no magnetic field present! Just that the forces of attraction in both directions are equal and balanced.

You can bias a reed switch to respond only to one pole of the magnet by adding a balancing magnet on the other side of the switch, to cancel the force of attraction to one pole and reinforce the attraction to the other pole. Again, you are not cancelling the _fields_, just the experienced forces from the fields.

If I suspend a weight by a rope, have I cancelled the field of gravity on the weight? Of course not, I have just balanced its force downward by another force upward.

[TinselKoala]

@TinselKoala,

"Buying a special magnet from, say, K&J Magnetics would be kind of silly, since the shipping cost alone will be quite a bit more than the cost of the magnet",

You're straight out of a tale from Charles Dickens you miserly thread ball Scrooge! Get a job!

Scrooge you, buddy. If you think it is an effective use of funds to pay 12 dollars to ship a 4 dollar magnet, that is your business and you are welcome to do it. My statement stands: It is kind of silly to do that, unless there is a very good reason to do so. And performing an experiment whose results you won't accept anyway, does not qualify as a very good reason.

[synchro1]

Scrooge you, buddy. If you think it is an effective use of funds to pay 12 dollars to ship a 4 dollar magnet, that is your business and you are welcome to do it. My statement stands: It is kind of silly to do that, unless there is a very good reason to do so. And performing an experiment whose results you won't accept anyway, does not qualify as a very good reason.

You have me howling you miserable "Skin Flint"! I tip my cleaning maid at least that much. You sound like you eat from trash cans! If you spent a fraction of the time you waste filling this forum up with useless verbosity on a part time job you could probably afford to eat more then dog kibble.

This is a quote from K&J:

USPS First Class Mail is a flat rate of $5. So that's a whole total of under ten bucks you're squealing about like a stuck pig!