Years ago an acquaintance took apart his Starrett magnetic base. When he
re-assembled it, it no longer worked, although the magnet still was one.
No parts were lost in the disassembly, and everything was replaced where it
belonged.

I witnessed the problem, so I know it's true. Can anyone explain why it no
longer functioned? If so, thanks!

Harold

On Tue, 1 Feb 2005 03:13:52 -0800, "Harold and Susan Vordos"
wrote:

Years ago an acquaintance took apart his Starrett magnetic base. When he
re-assembled it, it no longer worked, although the magnet still was one.
No parts were lost in the disassembly, and everything was replaced where it
belonged.

I witnessed the problem, so I know it's true. Can anyone explain why it no
longer functioned? If so, thanks!

Harold

This is because it uses Alnico type metal magnets
which must NEVER be removed from their associated pole
pieces (not even for milliseconds!) unless you have the
facilities to re-magnetise in situ after final assembly.

The characteristic that controls this is the
product of the magnet length and the material Coercive Force
- roughly speaking this controls the ability of the material
to retain much of it's magnetic strength during or after
exposure to an airgap in the magnetic circuit.

A magnet is open circuit if it's on the bench with
no pole pieces attached - the magnetic circuit is closed if
a continuous path through soft iron connects North pole to
South pole.

All the older types of magnet steels and even the
latest Alnicos behave this way and 50 to 90% loss of field
strength from incautious disassembly is possible.

Ceramic (ferrite) and rare earth (samarium etc)
magnets behave differently. Their coercive force is so high
that they can withstand being magnetically open circuited
without serious loss of field strength.

Jim

On Tue, 1 Feb 2005 03:13:52 -0800, the inscrutable "Harold and Susan
Vordos" spake:

Years ago an acquaintance took apart his Starrett magnetic base. When he
re-assembled it, it no longer worked, although the magnet still was one.
No parts were lost in the disassembly, and everything was replaced where it
belonged.

I witnessed the problem, so I know it's true. Can anyone explain why it no
longer functioned? If so, thanks!

SWAG: He didn't quite get the mag release assembled properly and
the magnet wasn't seating to the bottom when down.

As there are no wires, releasing of the Magic Smoke is -not- a
possibility here. My SWAG is interpreted from ownership of a fine
HF (Chiwanese) mag base (which has never been disassembled.)


---------------------------------------------------------------
Never put off 'til tomorrow | http://www.diversify.com
what you can avoid altogether. | Dynamic Website Applications
---------------------------------------------------------------

Harold and Susan Vordos wrote in message
...
Years ago an acquaintance took apart his Starrett magnetic base. When he
re-assembled it, it no longer worked, although the magnet still was one.
No parts were lost in the disassembly, and everything was replaced where
it
belonged.

I witnessed the problem, so I know it's true. Can anyone explain why it
no
longer functioned? If so, thanks!

Harold

Because the type of magnet used in it lost its magnetism very quickly on
dissassembly (low coercive force) when the pole pieces were removed. This
site explains about magnet properties and coercive force.

http://www.stanfordmagnets.com/magnet.html

On Tue, 01 Feb 2005 06:17:55 -0800, Larry Jaques novalidaddress@di wrote:
On Tue, 1 Feb 2005 03:13:52 -0800, the inscrutable "Harold and Susan
Vordos" spake:

Years ago an acquaintance took apart his Starrett magnetic base. When he
re-assembled it, it no longer worked, although the magnet still was one.
No parts were lost in the disassembly, and everything was replaced where it
belonged.

I witnessed the problem, so I know it's true. Can anyone explain why it no
longer functioned? If so, thanks!

SWAG: He didn't quite get the mag release assembled properly and
the magnet wasn't seating to the bottom when down.

Or, he put one of the magnet assemblies in backwards? That or the whole
flux-capacitor theory the other guy posted, that's good too. (???)

Larry answered cryptically: "SWAG: He didn't quite get the mag release
assembled properly and the magnet wasn't seating to the bottom when down."
And further, picquing our curiosity, he goes, "As there are no wires,
releasing of the Magic Smoke is -not- a possibility here. My SWAG is
interpreted from ownership of a fine HF (Chiwanese) mag base (which has
never been disassembled.)"

Now I, suspecting Larry was trying to interleague himself with co-responding
English friends "Jim" and Dave, looked up "SWAG". Sure 'nuf, there it was
in Newton's Telecom Dictionary: "Scientific Wild-Ass Guess".

Bob ( how to you make the grin symbol?) Swinney




"Larry Jaques" wrote in message
...
On Tue, 1 Feb 2005 03:13:52 -0800, the inscrutable "Harold and Susan
Vordos" spake:

Years ago an acquaintance took apart his Starrett magnetic base. When he
re-assembled it, it no longer worked, although the magnet still was one.
No parts were lost in the disassembly, and everything was replaced where
it
belonged.

I witnessed the problem, so I know it's true. Can anyone explain why it
no
longer functioned? If so, thanks!




---------------------------------------------------------------
Never put off 'til tomorrow | http://www.diversify.com
what you can avoid altogether. | Dynamic Website Applications
---------------------------------------------------------------

On Tue, 01 Feb 2005 11:53:54 +0000, the inscrutable
spake:

This is because it uses Alnico type metal magnets
which must NEVER be removed from their associated pole
pieces (not even for milliseconds!) unless you have the
facilities to re-magnetise in situ after final assembly.

Alnicos are semi-permanent magnets? Interesting. I'll have
to read more about them. I'd never heard of them.


---------------------------------------------------------------
Never put off 'til tomorrow | http://www.diversify.com
what you can avoid altogether. | Dynamic Website Applications
---------------------------------------------------------------

Larry Jaques wrote:

On Tue, 01 Feb 2005 11:53:54 +0000, the inscrutable
spake:


This is because it uses Alnico type metal magnets
which must NEVER be removed from their associated pole
pieces (not even for milliseconds!) unless you have the
facilities to re-magnetise in situ after final assembly.


Alnicos are semi-permanent magnets? Interesting. I'll have
to read more about them. I'd never heard of them.

They're permanent, just not forgiving. It was _the_ magnetic material
until the hard-ferrite ceramics came about.

AlNiCo can be cast, and magnetized in place (this is the exact flip side
of being demagnetized in place), it requires a lot of length to develop
the coercivity necessary to cross an airgap.

Ferrite can be mass-produced cheaply, it's much less likely to
demagnetize than AlNiCo, but it _can_ be demagnetized if you run too
much current in a DC motor with ferrite magnets.

Neodymium-Iron-Boron is tough stuff, "supermagnet" material. Its strong
and nearly (magnetically) indestructible. It tends to temporarily loose
magnetization with elevated temperatures; AlNiCo beats it out there.
Physically it corrodes readily and it's very brittle, and if you let two
magnets whang into each other the impact can shatter one or both magnets
and let little sharp bits of shrapnel fly around.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

In article ,
says...

This is because it uses Alnico type metal magnets
which must NEVER be removed from their associated pole
pieces (not even for milliseconds!) unless you have the
facilities to re-magnetise in situ after final assembly.

Does this mean that turning the base to the "on"
position when it is not in contact with a surface
to complete the magnetic circuit, will ruin it?

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

On Tue, 01 Feb 2005 11:53:54 +0000, the inscrutable
spake:

This is because it uses Alnico type metal magnets
which must NEVER be removed from their associated pole
pieces (not even for milliseconds!) unless you have the
facilities to re-magnetise in situ after final assembly.

That's dibble and fluff. Alnico magnets do decay, albeit slowly (over
years). But no magnetic base uses a "keeper" while it's in use (none I know
of have a keeper, at all), or it wouldn't stick to anything. The keeper is
designed to _complete_ the flux circuit, in order to lengthen the life of
the magnet. If the circuit were completed with a keeper (shunting the field
between the poles), the magnet couldn't stick to your work surface.

He's confused.

More likely, anyway (if the base is relatively new - say twenty years or
less), it has a ceramic/ferrite magnet. They hardly decay in strength at
all, with or without a keeper.

LLoyd

Jim sez: "Does this mean that turning the base to the "on" position when it
is not in contact with a surface to complete the magnetic circuit, will ruin
it?"

Naw! "Leakage" magnetic effect would still practically emulate a "keeper".

Bob Swinney

"jim rozen" wrote in message
...
In article ,

says...

This is because it uses Alnico type metal magnets
which must NEVER be removed from their associated pole
pieces (not even for milliseconds!) unless you have the
facilities to re-magnetise in situ after final assembly.


Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

wrote in message
...
snip----

This is because it uses Alnico type metal magnets
which must NEVER be removed from their associated pole
pieces (not even for milliseconds!) unless you have the
facilities to re-magnetise in situ after final assembly.

The characteristic that controls this is the
product of the magnet length and the material Coercive Force
- roughly speaking this controls the ability of the material
to retain much of it's magnetic strength during or after
exposure to an airgap in the magnetic circuit.

A magnet is open circuit if it's on the bench with
no pole pieces attached - the magnetic circuit is closed if
a continuous path through soft iron connects North pole to
South pole.

All the older types of magnet steels and even the
latest Alnicos behave this way and 50 to 90% loss of field
strength from incautious disassembly is possible.

Ceramic (ferrite) and rare earth (samarium etc)
magnets behave differently. Their coercive force is so high
that they can withstand being magnetically open circuited
without serious loss of field strength.

Jim

Thanks, Jim. You likely hit the answer right on the head. This incident
occured back in ealy '67----when mag bases were, indeed, Alnico.

Harold

On 1 Feb 2005 11:27:56 -0800, jim rozen
wrote:

In article ,
says...

This is because it uses Alnico type metal magnets
which must NEVER be removed from their associated pole
pieces (not even for milliseconds!) unless you have the
facilities to re-magnetise in situ after final assembly.

Does this mean that turning the base to the "on"
position when it is not in contact with a surface
to complete the magnetic circuit, will ruin it?

Jim


The "associated pole pieces" are the soft iron
parts of the magnetic base.

The geometry and physical arrangement of these pole
pieces is carefully chosen so that there is just sufficient
alternate path (i e leakage flux) to enable the magnet to
operate close to it's maximum energy point when NOT in
contact with an iron surface. This is the BH max point - it
corresponds to the most efficient use of the magnet material
Because of this, the magnet is not degraded when left 'ON'
and not in contact with an iron surface. This deliberate
leakage flux path acts as carefully chosen partial 'keeper'
to protect the magnet from the demagntising effect of a
wholly open magnetic circuit.

When in close contact with an iron surface the overall
magnetic circuit reluctance is very much lower and almost
all the available flux is then diverted away from the
designed in leakage path and now flows through the pole
pieces into the work surface.

Jim

On Tue, 01 Feb 2005 19:44:47 GMT, "Lloyd E. Sponenburgh"
wrote:


On Tue, 01 Feb 2005 11:53:54 +0000, the inscrutable
spake:

This is because it uses Alnico type metal magnets
which must NEVER be removed from their associated pole
pieces (not even for milliseconds!) unless you have the
facilities to re-magnetise in situ after final assembly.

That's dibble and fluff. Alnico magnets do decay, albeit slowly (over
years). But no magnetic base uses a "keeper" while it's in use (none I know
of have a keeper, at all), or it wouldn't stick to anything. The keeper is
designed to _complete_ the flux circuit, in order to lengthen the life of
the magnet. If the circuit were completed with a keeper (shunting the field
between the poles), the magnet couldn't stick to your work surface.

He's confused.

More likely, anyway (if the base is relatively new - say twenty years or
less), it has a ceramic/ferrite magnet. They hardly decay in strength at
all, with or without a keeper.

LLoyd

The decay rate of Alnico magnets in a properly
designed assembly is negligible. A familiar example is the
use of Alnico magnets in moving coil meters which directly
rely on the long term stability of the magnetic field
strength.

The leakage function of the pole pieces in
partially closing the magnetic circuit is covered in my
reply to the valid query raised by Jim Rozen.

Pentagrid

On Tue, 01 Feb 2005 23:15:04 +0000, Mark Rand
wrote:

On Tue, 1 Feb 2005 03:13:52 -0800, "Harold and Susan Vordos"
wrote:

Years ago an acquaintance took apart his Starrett magnetic base. When he
re-assembled it, it no longer worked, although the magnet still was one.
No parts were lost in the disassembly, and everything was replaced where it
belonged.

I witnessed the problem, so I know it's true. Can anyone explain why it no
longer functioned? If so, thanks!

Harold


As a corollary to all the comments, Anyone have an idea how many thousand
ampere turns I might need to re-magnetise an old mag base after building a
good core around it?


Mark Rand
RTFM

Alnico magnets need about 8,000 ampere turns per inch
length of magnet to fully re-magnetise. This assumes that
you've arranged suitable lumps of soft iron or mild steel to
form a closed magnetic circuit with one leg of the coil
passing through the middle.

The current needs only to be present for a few
milliseconds.

Jim

On Tue, 1 Feb 2005 03:13:52 -0800, "Harold and Susan Vordos"
wrote:

Years ago an acquaintance took apart his Starrett magnetic base. When he
re-assembled it, it no longer worked, although the magnet still was one.
No parts were lost in the disassembly, and everything was replaced where it
belonged.

I witnessed the problem, so I know it's true. Can anyone explain why it no
longer functioned? If so, thanks!

Harold


As a corollary to all the comments, Anyone have an idea how many thousand
ampere turns I might need to re-magnetise an old mag base after building a
good core around it?


Mark Rand
RTFM

"Harold and Susan Vordos" wrote in message
...
All the older types of magnet steels and even the
latest Alnicos behave this way and 50 to 90% loss of field
strength from incautious disassembly is possible.

Ceramic (ferrite) and rare earth (samarium etc)
magnets behave differently. Their coercive force is so high
that they can withstand being magnetically open circuited
without serious loss of field strength.

Jim

Thanks, Jim. You likely hit the answer right on the head. This incident
occured back in ealy '67----when mag bases were, indeed, Alnico.

Harold

Nope, he's completely off-base. Totally confused.
LLoyd

In article ,
Harold and Susan Vordos wrote:
Years ago an acquaintance took apart his Starrett magnetic base. When he
re-assembled it, it no longer worked, although the magnet still was one.
No parts were lost in the disassembly, and everything was replaced where it
belonged.

I witnessed the problem, so I know it's true. Can anyone explain why it no
longer functioned? If so, thanks!

The magnet needs a "keeper" to keep the magnetic flux high. The
body works as one normally. When you pull the magnet out, you have to
slide it into a mild steel cylinder as it exits the one in the base.

It should be possible to re "pole" the magnet with a serious
magnetic pulse applied to the pole pieces of the base once it is
reassembled and switched to the "on" position. But it is probably
cheaper to just buy a new one, if you don't have the materials on hand
to build a device to "pole" the magnet.

Now to see what others have said. :-)

Enjoy,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

On Tue, 01 Feb 2005 23:15:04 +0000, Mark Rand wrote:
On Tue, 1 Feb 2005 03:13:52 -0800, "Harold and Susan Vordos"
wrote:

Years ago an acquaintance took apart his Starrett magnetic base. When he
re-assembled it, it no longer worked, although the magnet still was one.
No parts were lost in the disassembly, and everything was replaced where it
belonged.

I witnessed the problem, so I know it's true. Can anyone explain why it no
longer functioned? If so, thanks!

Harold


As a corollary to all the comments, Anyone have an idea how many thousand
ampere turns I might need to re-magnetise an old mag base after building a
good core around it?

The copper involved would probably cost more than a new mag base.

On Wed, 02 Feb 2005 00:21:26 GMT, "Lloyd E. Sponenburgh"
wrote:


Nope, he's completely off-base. Totally confused.
LLoyd

He's right on. Trolling?

On 2 Feb 2005 18:23:27 GMT, Dave Hinz wrote:

On Tue, 01 Feb 2005 23:15:04 +0000, Mark Rand wrote:
On Tue, 1 Feb 2005 03:13:52 -0800, "Harold and Susan Vordos"
wrote:

Years ago an acquaintance took apart his Starrett magnetic base. When he
re-assembled it, it no longer worked, although the magnet still was one.
No parts were lost in the disassembly, and everything was replaced where it
belonged.

I witnessed the problem, so I know it's true. Can anyone explain why it no
longer functioned? If so, thanks!

Harold


As a corollary to all the comments, Anyone have an idea how many thousand
ampere turns I might need to re-magnetise an old mag base after building a
good core around it?

The copper involved would probably cost more than a new mag base.

Depends mostly on the internal airgaps. Figure about 30 amp-turns
per .001" of total airgap to saturate the steel parts and the alnico.
(That's 15,000 oersted) Non-magnetic material (brass or aluminum) in
the flux path counts as airgap. You can get several hundred amps out
of an auto battery for the short time it would take to blow the wire.
#16 or #14 wire might be about right; it'd fuse in a second or two
but the job would be long done by then. 50 turns of #16 should get
its attention. More turns of finer wire might work too, but take
more winding.

I won't strongly assert that this will work, but it's easy to try.

Mark Rand wrote:
As a corollary to all the comments, Anyone have an idea how many thousand
ampere turns I might need to re-magnetise an old mag base after building a
good core around it?

I tried re-magnetizing a base a while ago. I screwed around with it a
lot and never did get very good results. I dumped charged capacitors to
get the large currents. It is definitely not worth the effort, but I
knew that and was just doing it for kicks, er ... science, yeah
"science".

I used the wisdom of this group and posted about it. Google-group the
RCM archives on "magnetic base", author "engelhardt".

Bob

On Tue, 01 Feb 2005 11:13:13 -0800, the inscrutable Tim Wescott
spake:

Larry Jaques wrote:

On Tue, 01 Feb 2005 11:53:54 +0000, the inscrutable
spake:


This is because it uses Alnico type metal magnets
which must NEVER be removed from their associated pole
pieces (not even for milliseconds!) unless you have the
facilities to re-magnetise in situ after final assembly.


Alnicos are semi-permanent magnets? Interesting. I'll have
to read more about them. I'd never heard of them.

They're permanent, just not forgiving. It was _the_ magnetic material
until the hard-ferrite ceramics came about.

To clarify, I was aware of alnicos (most speakers had them--for most
of my pre-teen and teen years, anyway) but wasn't aware of the
possibility of demagnetizing any "permanent" magnets.


AlNiCo can be cast, and magnetized in place (this is the exact flip side
of being demagnetized in place), it requires a lot of length to develop
the coercivity necessary to cross an airgap.

Ferrite can be mass-produced cheaply, it's much less likely to
demagnetize than AlNiCo, but it _can_ be demagnetized if you run too
much current in a DC motor with ferrite magnets.

Neodymium-Iron-Boron is tough stuff, "supermagnet" material. Its strong
and nearly (magnetically) indestructible. It tends to temporarily loose
magnetization with elevated temperatures; AlNiCo beats it out there.
Physically it corrodes readily and it's very brittle, and if you let two
magnets whang into each other the impact can shatter one or both magnets
and let little sharp bits of shrapnel fly around.

I have some tiny NIB mags which are strong enough to give my fingers
blood blisters when I'm careless with 'em. Amazing. I wish I'd had
them during experiments when I was a small child.


--
The clear and present danger of top-posting explored at:
http://www.netmeister.org/news/learn2quote2.html
------------------------------------------------------
http://diversify.com Premium Website Development

On Wed, 02 Feb 2005 16:20:30 -0500, Bob Engelhardt
wrote:

Mark Rand wrote:
As a corollary to all the comments, Anyone have an idea how many thousand
ampere turns I might need to re-magnetise an old mag base after building a
good core around it?

I tried re-magnetizing a base a while ago. I screwed around with it a
lot and never did get very good results. I dumped charged capacitors to
get the large currents. It is definitely not worth the effort, but I
knew that and was just doing it for kicks, er ... science, yeah
"science".

I used the wisdom of this group and posted about it. Google-group the
RCM archives on "magnetic base", author "engelhardt".

Bob

If you want an economical method of re-magnetising a
useful variant of the old little gem fuse blower trick works
like a charm.

For the first quarter cycle of the 50/60 Hz
supply frequency the ordinary domestic supply will deliver
a peak current many times the rated current of the primary
fuse or circuit breaker without discommoding the fuse or
tripping the breaker.

A quarter cycle is ample time for a magnetising pulse.

In the UK, domestic 240v ring mains are
typically 30A fuse or breaker protected and feed a number of
sockets each rated to deliver 13A.

Plugs for these sockets can be fitted with a 3,
5, or 13A fuse. A 13A fused plug delivers a peak current of
over 500A into a dead short in the instant before the fuse
ruptures so that many tens of kilowatts of peak power is
available. This is conveniently switched on by inserting the
plug and safely switched off by rupture of the fuse.

I've no experience of North American supplies
but it's reasonable to expect that a similar technique is
workable.

The magnetising coil can be fairly elementary.
There is a sizeable heat pulse into the coil but, provided
the wire thickness is about twice as thick as the protecting
fuse wire thickness, it doesn't get hot enough to matter.
For small magnets ten or twenty turns of ordinary hookup
wire is all that is needed.

Two diodes are needed to control the polarity
of the pulse and to damp out transient current reversals.

The first diode is series connected anode to supply
live, cathode to coil. The second diode is shunted across
the coil, cathode to hot end (i.e.cathode to cathode). The
series diode controls the polarity of the current pulse. The
shunt diode acts as a reverse diode and provides a safe path
for the inductive overswing which occurs at the abrupt
cessation of the input current pulse.

Surprisingly small diodes are adequate because of
the very high non-repetitive single pulse current ratings of
most power rectifiers. Motorola MR754 is typical. Although
nominally a 6amp rectifier its single pulse peak current
rating is 400A for 8mS!

Theoretically you should carefully choose the
resitance and inductance of the magnetising coil to achieve
maximum magnetising capability within the peak current
capabilities of the rectifiers. In practice, the available
peak power is so high that it is sufficient to just cram
turns into the available space and to ensure that, including
connecting wire, a total of at least 20 ft of wire is used.

Jim

In article ,
says...

The geometry and physical arrangement of these pole
pieces is carefully chosen so that there is just sufficient
alternate path (i e leakage flux) to enable the magnet to
operate close to it's maximum energy point when NOT in
contact with an iron surface. This is the BH max point - it
corresponds to the most efficient use of the magnet material
Because of this, the magnet is not degraded when left 'ON'
and not in contact with an iron surface. This deliberate
leakage flux path acts as carefully chosen partial 'keeper'
to protect the magnet from the demagntising effect of a
wholly open magnetic circuit.

When in close contact with an iron surface the overall
magnetic circuit reluctance is very much lower and almost
all the available flux is then diverted away from the
designed in leakage path and now flows through the pole
pieces into the work surface.

Excellent explaination, thank you, Jim.

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

"Larry Jaques" wrote in message
...
To clarify, I was aware of alnicos (most speakers had them--for most
of my pre-teen and teen years, anyway) but wasn't aware of the
possibility of demagnetizing any "permanent" magnets.


I'm in my late fifties. I have a fairly large collection of odd-n-ends
alnico magnets from speaker pull-outs I've kept since my teens.

They're all weaker than they were, but they're all still magnets, and still
useful for holding drawings, etc.

To those insisting that alnico magnets lose their strength almost instantly
in the absence of a keepe, consider: Millions of kitchen pot holders were
made in the fifties, sixties, and seventies with a small 3/16^2 x 3/4"
alnico bar magnet within the hanging loop (for hanging the thing on your
fridge or oven door).

They were made, shipped, stored, and usually used in the home without any
keeper of any sort -- usually when thrown in a drawer, they didn't even have
the benefit of being near a ferrous substance. But they kept their
strength - enough to hang the things up - for _years_.

LLoyd

On Thu, 03 Feb 2005 13:01:39 GMT, "Lloyd E. Sponenburgh"
wrote:


"Larry Jaques" wrote in message
.. .
To clarify, I was aware of alnicos (most speakers had them--for most
of my pre-teen and teen years, anyway) but wasn't aware of the
possibility of demagnetizing any "permanent" magnets.


I'm in my late fifties. I have a fairly large collection of odd-n-ends
alnico magnets from speaker pull-outs I've kept since my teens.

They're all weaker than they were, but they're all still magnets, and still
useful for holding drawings, etc.

To those insisting that alnico magnets lose their strength almost instantly
in the absence of a keepe, consider: Millions of kitchen pot holders were
made in the fifties, sixties, and seventies with a small 3/16^2 x 3/4"
alnico bar magnet within the hanging loop (for hanging the thing on your
fridge or oven door).

They were made, shipped, stored, and usually used in the home without any
keeper of any sort -- usually when thrown in a drawer, they didn't even have
the benefit of being near a ferrous substance. But they kept their
strength - enough to hang the things up - for _years_.

LLoyd

Just because they shipped magnets without keepers doesn't mean that
they are at their fullest possible strength. And if alnicos they
weren't. I've seen the mag base effect where taking one apart weakens
the magnet. Curious, I asked an engineer I know about it and he told
me what had happened. A similar effect can be seen in stepper motors.
They are assembled and then magnetised. If the motor is taken apart it
will lose much of it's torque. I've been told that some of the newer
motors are not as sensitive to this. Even if this is true, the motors
I modify on a regular still come with instructions saying not to
remove the rotor. The rotors are not alnico though.
ERS

On Thu, 03 Feb 2005 13:01:39 GMT, "Lloyd E. Sponenburgh"
wrote:

I'm in my late fifties. I have a fairly large collection of odd-n-ends
alnico magnets from speaker pull-outs I've kept since my teens.

They're all weaker than they were, but they're all still magnets, and still
useful for holding drawings, etc.

To those insisting that alnico magnets lose their strength almost instantly
in the absence of a keepe, consider: Millions of kitchen pot holders were
made in the fifties, sixties, and seventies with a small 3/16^2 x 3/4"
alnico bar magnet within the hanging loop (for hanging the thing on your
fridge or oven door).

They were made, shipped, stored, and usually used in the home without any
keeper of any sort -- usually when thrown in a drawer, they didn't even have
the benefit of being near a ferrous substance. But they kept their
strength - enough to hang the things up - for _years_.

The flux density in those alnico household magnets is considerably
less than that of saturated alnico. It became so almost instantly
when removed from the magnetizer at the factory, before it ever
arrived at the store.

In article , Don Foreman says...

The flux density in those alnico household magnets is considerably
less than that of saturated alnico. It became so almost instantly
when removed from the magnetizer at the factory, before it ever
arrived at the store.

Anyone who works on magnetos knows that the rotors should
always be stored with 'keepers' on the pole pieces.

I always wondered what would happen if I took one of those
old mags, and set it up with some of the new supermagnets.

Jim


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please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
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On 3 Feb 2005 09:53:47 -0800, jim rozen
wrote:

In article , Don Foreman says...

The flux density in those alnico household magnets is considerably
less than that of saturated alnico. It became so almost instantly
when removed from the magnetizer at the factory, before it ever
arrived at the store.

Anyone who works on magnetos knows that the rotors should
always be stored with 'keepers' on the pole pieces.

I always wondered what would happen if I took one of those
old mags, and set it up with some of the new supermagnets.

Jim


The keeper discussion still seems to be simmering
gently so a few extra comments might be helpful.

For the moment consider only the iron alloy based
magnets. This includes Alnico, Alni and the earlier alloys
using cobalt,tungsten or carbon additives.

These alloys, when magnetised to saturation in a
fully closed magnetic circuit magnetising jig, all retain
about the same flux density after the magnetising field is
removed. This is quoted in the manufacturers literature as
the B remanence figure. It is typically about 12 Kilogauss.

If, now, a small air gap is introduced there is
an immediate drop in flux density which is only partially
recovered when the air gap is returned to zero.

Provided that any later experimentally
introduced airgap is smaller than this first gap there is no
further degradation.

If however a subsequent air gap is greater than
any preceding gap it ratchets the closed magnetic circuit
flux density down further and establishes a new range of
airgaps that the magnet can accept without further
degradation.

This is the behaviour that we
observe when we disassemble a magnetic base or remove the
armature from a servomotor that uses this type of magnet.

It doesn't explain the role of keepers commonly
used to protect magnets when not in use. Once a magnet has
been exposed to the demagnetising influence of a large
airgap, putting the keeper back on will NOT restore it to
it's previous state.

The keeper is there for a different reason.
Permanent magnets exhibit their magnetism as a result of the
saturating magnetising field forcing their randomly oriented
internal magnetic domains into a nice orderly additive
arrangement. As noted above this is fairly easily disturbed
by the demagnetising effect of an airgap.

It is also disturbed by mechanical shock and it's
sensitivity to shock or stray external magnetic fields is
increased if it's also fighting with the demagnetising
influence of an airgap. The older permanent magnet materials
which have low intrinsic coercive force are particularly
sensitive to this problem and this is why you see the old
schoolboy bar magnets and ancient magneto horshoe magnets
religiously stored with keepers on when not in use.

A keeper is also used occasionally to provide a
temporary alternate flux path when it is necessary to remove
part of the normal main flux path. This avoids exposing the
magnet to the demagnetising effect of a large disasembled
airgap. The keeper must, of course, be installed BEFORE the
main flux path is removed.

The mechanical shock effect is easily
demonstrated in the workshop. Magnetise a screwdriver or a
piece of hardened carbon steel by stroking it against a
decent permanent magnet. Left undisturbed on the bench it
will retain it's newfound magnetism indefinitely. Bash it
hard against any sustantial lump of metal and much of its
magnetism is immediately destroyed.

Similar effects, but to a much smaller degree
occur with ferrite and the rare earth supermagnets. With
most designs, additional airgap induced degradation is not
enough to matter.

Designs using the old magnet materials can be
uprated by changing to rare earth magnets but the vastly
different optimum length to diameter ratio of the new
material means that a pretty major mechanical redesign is
necessary.

Jim

Very nice explanation, thank you. I've learned so much in this NG! Bob