Anyone know how these old singer foot pedals work? There's not much
to this thing, but I can't for the life of me figure out how it's
supposed to work!

The one I have contains a long ceramic tube filled with a stack or
maybe 100 graphite disks. One end connects to the sewing machine
motor (I assume), and at the other end there's a contact that's
brought closer to and eventually touching the contact on the end of
the tube as the foot pedal is depressed.

The foot pedal was dropped and I'm trying to repair it, but so far I
can't get any variable speed out of it - I get either off (when the
contact isn't touching the end of the graphite-disc-tube) or ON-HIGH
when the contact touches the end of the graphite-disc-tube.

Sorry for the miserable description - it's been about a month since I
had the thing apart - just figured I'd post here to see if anyone had
any suggestions.

thanks!

-Pete

On 4/22/2009 1:17 PM spake thus:

Anyone know how these old singer foot pedals work? There's not much
to this thing, but I can't for the life of me figure out how it's
supposed to work!

The one I have contains a long ceramic tube filled with a stack or
maybe 100 graphite disks. One end connects to the sewing machine
motor (I assume), and at the other end there's a contact that's
brought closer to and eventually touching the contact on the end of
the tube as the foot pedal is depressed.

The foot pedal was dropped and I'm trying to repair it, but so far I
can't get any variable speed out of it - I get either off (when the
contact isn't touching the end of the graphite-disc-tube) or ON-HIGH
when the contact touches the end of the graphite-disc-tube.

Sorry for the miserable description - it's been about a month since I
had the thing apart - just figured I'd post here to see if anyone had
any suggestions.

That sounds like a real old-timey one.

They're just rheostats. If you're intent on fixing the one you have,
rotsa ruck: that's a restoration job that may require materials (e.g.,
graphite discs) not easily available anymore. It sounds like there
should be some intermediate contact points between the ends of the
ceramic tube that make for variable resistance; see anything like that?
Obviously, the ceramic, which is an insulator, isn't going to allow any
such contact.

If you just wanted to get it working you could use a more modern foot
control, which is also just a rheostat but usually using resistance wire
instead of carbon objects.

Have you tried poking around the graphite pieces with an ohmmeter? You
could see if maybe there's a break in continuity (electrical) somewhere
that's making it go all-or-none.


--
Save the Planet
Kill Yourself

- motto of the Church of Euthanasia (http://www.churchofeuthanasia.org/)

wrote:

Anyone know how these old singer foot pedals work? There's not much
to this thing, but I can't for the life of me figure out how it's
supposed to work!

The one I have contains a long ceramic tube filled with a stack or
maybe 100 graphite disks. One end connects to the sewing machine
motor (I assume), and at the other end there's a contact that's
brought closer to and eventually touching the contact on the end of
the tube as the foot pedal is depressed.

The foot pedal was dropped and I'm trying to repair it, but so far I
can't get any variable speed out of it - I get either off (when the
contact isn't touching the end of the graphite-disc-tube) or ON-HIGH
when the contact touches the end of the graphite-disc-tube.

The contact resistance between the carbon discs is varied by the
pressure on the stack (in a similar way to the action of a carbon
microphone). There will be some sort of compression spring between the
operating lever and the top of the stack (it might be a leaf spring).
When the spring is almost fully compressed, a contact by-passes the
resistance stack and gives full power.

It sounds as though the compression system isn't functioning or the
by-pass contacts are closing too quickly. You need to identify an
adjustment screw somewhere in the linkage to the compression spring and
set up the pressure on the stack so that the compression is minimal when
contact is first made with the stack and increases until the motor is
nearly at full power before the by-pass contacts close.

I can supply a photograph of the inside of the controller for a 1950s
Model 201K (for 150 to 250 volt operation) if that would help you.


--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

lid (Adrian Tuddenham) writes:

wrote:

Anyone know how these old singer foot pedals work? There's not much
to this thing, but I can't for the life of me figure out how it's
supposed to work!

The one I have contains a long ceramic tube filled with a stack or
maybe 100 graphite disks. One end connects to the sewing machine
motor (I assume), and at the other end there's a contact that's
brought closer to and eventually touching the contact on the end of
the tube as the foot pedal is depressed.

The foot pedal was dropped and I'm trying to repair it, but so far I
can't get any variable speed out of it - I get either off (when the
contact isn't touching the end of the graphite-disc-tube) or ON-HIGH
when the contact touches the end of the graphite-disc-tube.

The contact resistance between the carbon discs is varied by the
pressure on the stack (in a similar way to the action of a carbon
microphone). There will be some sort of compression spring between the
operating lever and the top of the stack (it might be a leaf spring).
When the spring is almost fully compressed, a contact by-passes the
resistance stack and gives full power.

It sounds as though the compression system isn't functioning or the
by-pass contacts are closing too quickly. You need to identify an
adjustment screw somewhere in the linkage to the compression spring and
set up the pressure on the stack so that the compression is minimal when
contact is first made with the stack and increases until the motor is
nearly at full power before the by-pass contacts close.

I can supply a photograph of the inside of the controller for a 1950s
Model 201K (for 150 to 250 volt operation) if that would help you.

It may also be possible to simply buy a replacement foot-pedal rheostat
but it would have to be matched up in terms of resistance range.

Of course, if you want authenticity, you'll have to repair it!

--
sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

No one has said anything about replacing the pot with an SCR or triac
controller.

On Thu, 23 Apr 2009 04:20:31 -0700, "William Sommerwerck"
wrote:

No one has said anything about replacing the pot with an SCR or triac
controller.

Sacrilege! It's probably an antique Singer sewing machine. Trends
towards modernization of antiques are highly discouraged by convention
among collectors. The desecrations of cherished antiques never seems
to end. The slippery slope of modernization and evil upgrading will
soon lead to the total replacement of the entire sewing machine. Such
things cannot be tolerated. Maintaining an old machine is an art.
Doing the same to a modern plastic equivalent is often a waste of
time. Best to restore the old Singer to its original condition than
to butcher it into some manner of hybrid contrivance.

It's a marginal idea anyway. Small sewing machine motors use solid
carbon brushes. That's fine for low duty cycle, variable speed, and
minimal load motors. Add an electronic speed control and the brushes
now see a higher frequency component in the wave form. The result is
a slight increase in arcing of the brushes, which causes increased
wear of the copper commutator sections. Modern brushes are a sandwich
of graphite (for lubrication) and carbon (for conductivity) to prevent
this type of wear. My guess(tm) is that an electronic speed control
will take about 25% off the life of the motor.

Some instructions and clues on rebuilding sewing machine motors:
http://reviews.ebay.com/HOW-TO-OVERHAUL-YOUR-SEWING-MACHINE-MOTOR-IN-30-MINUTES_W0QQugidZ10000000004665359
"The next step is to clean the armature commutator. This is where
the carbon brushes make contact. 90% of motor's troubles comes
from a bad commutator. Heat, oil, carbon accumulation and dirt
are the ideal mix to provocate(sic) sparkling when electricity
is applied to the motor. Under load, the sparkling will increase
and some motors will start smoking..."

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Wow - nice to see there's still activity on this list, and thanks for
all the replies!

KISS is my driver. I'd like to keep it simple, and cheap. Fixing the
existing part (if possible) is nearly always my first choice. Besides,
I no longer have access to my bench and most of my equipment - all I
have here is an old Weller soldering iron, some solder, my favorite
Tek scope, and a box full of parts I needed to fix something recently.
All the rest is in storage

Anyway, here's a shot of the pedal's guts, with some labels to help
clarify.

http://i151.photobucket.com/albums/s...y/P4231021.jpg

1. Spring and Linkage. This is pressed downward when the pedal is
depressed.
2. Brass Linkage/Contact. This moves to the right via linkage 1 when
the pedal is depressed.
3. Copper Contact. This moves to the right and toward the graphite
contact on the left end of the Carbon Pile when the pedal is
depressed.
4. Carbon Pile. This is a long ceramic tube filled with ~100 graphite
discs and a graphite contact on either end.
5. Bypass Contact. When the pedal is fully depressed, the Brass
Linkage touches this contact and bypasses the Carbon Pile.

Now - here's what I didn't mention previously... I got this thing in
pieces at a garage sale, along with the sewing machine. The previous
owner had dropped the pedal, cracking the bakelite and spilling its
innards all over. He assured me nothing was missing, and when I
fiddled with it, it all seemed to fit into place (as shown). BUT, I'm
not 100% sure I'm not missing something, and I'm also not 100% sure I
have it assembled correctly. The Copper Contact was as shown (nearly
straight) when I got it - it doesn't make contact with the Carbon Pile
until the Brass Linkage is nearly touching the Bypass Contact. Also,
the slot where the Copper Contact and Brass Linkage tie together is
held together very tenuously - I'm afraid it's going to fall apart
every time I fully-depress the pedal.

So what I could use here is an evaluation of the parts and placements
- do I have everything? Is it all in the right place? I can't figure
any other way to set it up. Any other thoughts?

Thanks all!

-Pete

On 4/23/2009 10:12 AM spake thus:

Anyway, here's a shot of the pedal's guts, with some labels to help
clarify.

http://i151.photobucket.com/albums/s...y/P4231021.jpg

1. Spring and Linkage. This is pressed downward when the pedal is
depressed.
2. Brass Linkage/Contact. This moves to the right via linkage 1 when
the pedal is depressed.
3. Copper Contact. This moves to the right and toward the graphite
contact on the left end of the Carbon Pile when the pedal is
depressed.
4. Carbon Pile. This is a long ceramic tube filled with ~100 graphite
discs and a graphite contact on either end.
5. Bypass Contact. When the pedal is fully depressed, the Brass
Linkage touches this contact and bypasses the Carbon Pile.

Now - here's what I didn't mention previously... I got this thing in
pieces at a garage sale, along with the sewing machine. The previous
owner had dropped the pedal, cracking the bakelite and spilling its
innards all over. He assured me nothing was missing, and when I
fiddled with it, it all seemed to fit into place (as shown). BUT, I'm
not 100% sure I'm not missing something, and I'm also not 100% sure I
have it assembled correctly. The Copper Contact was as shown (nearly
straight) when I got it - it doesn't make contact with the Carbon Pile
until the Brass Linkage is nearly touching the Bypass Contact. Also,
the slot where the Copper Contact and Brass Linkage tie together is
held together very tenuously - I'm afraid it's going to fall apart
every time I fully-depress the pedal.

So what I could use here is an evaluation of the parts and placements
- do I have everything? Is it all in the right place? I can't figure
any other way to set it up. Any other thoughts?

Pretty clear how it works now; as someone described up-thread, the pile
of carbon discs gets compressed, reducing their resistance. So obviously
this happens when the spring on the left (3) pushes on the metal stud at
the left end of the ceramic tube.

So apparently the problem is either that the carbon discs aren't making
enough contact with each other inside the tube, or are contaminated, or
that there's not enough pressure being exerted on the left end of the
tube by that spring. Which could mean that either it's not put together
correctly, that there's a part missing (though I can't see where), or
that something needs bending or some other adjustment to work correctly.


--
Save the Planet
Kill Yourself

- motto of the Church of Euthanasia (http://www.churchofeuthanasia.org/)

wrote:

[...]

It looks as though you hve got everything in the right place

[...]
The Copper Contact was as shown (nearly
straight) when I got it - it doesn't make contact with the Carbon Pile
until the Brass Linkage is nearly touching the Bypass Contact.

That's why you aren't getting a gradual start-up, the resistance isn't
coming into play until too late in the operating sequence.

Also,
the slot where the Copper Contact and Brass Linkage tie together is
held together very tenuously - I'm afraid it's going to fall apart
every time I fully-depress the pedal.

Gently curving the 'copper' (more likely phosphor bronze) strip towards
the pile would cause it to make contact earlier and would also change
the angle of the fork so that it held the brass linkage more positively.
Perhaps the fork should be angled more sharply to make sure it is well
hooked into the strip (be careful not to crease the metal and snap it).

Have you checked the carbon discs to make sure that some of them aren't
shattered - or some might be missing?. That would make the pile too
short to touch the 'copper' strip properly.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

On Thu, 23 Apr 2009 10:12:49 -0700 (PDT), wrote:

3. Copper Contact. This moves to the right and toward the graphite
contact on the left end of the Carbon Pile when the pedal is
depressed.

I would try adding some (conducting) padding at the centre of the bronze spring
(around the "3" area) to make contact with the carbon. Is it possible a
copper/brass disc is missing around there?

--
Geo

On Wednesday, April 22, 2009 3:17:37 PM UTC-5, wrote:
Anyone know how these old singer foot pedals work? There's not much
to this thing, but I can't for the life of me figure out how it's
supposed to work!

The one I have contains a long ceramic tube filled with a stack or
maybe 100 graphite disks. One end connects to the sewing machine
motor (I assume), and at the other end there's a contact that's
brought closer to and eventually touching the contact on the end of
the tube as the foot pedal is depressed.

The foot pedal was dropped and I'm trying to repair it, but so far I
can't get any variable speed out of it - I get either off (when the
contact isn't touching the end of the graphite-disc-tube) or ON-HIGH
when the contact touches the end of the graphite-disc-tube.

Sorry for the miserable description - it's been about a month since I
had the thing apart - just figured I'd post here to see if anyone had
any suggestions.

thanks!

-Pete

Just fixed one of these and found this ancient thread. Thought I might put this up for people finding this thread. The carbon discs get burned, so go through the stack and discard any that have broken, then take a very fine sand paper (600 grit)and lightly sand both faces of each disc. Make up the space for any discards or accidentally broken while sanding discs with some small washers. It is important that the gap be filled with something strudy, not foil or steel wool. We don't want springinees to the stack. The discs are poor conductors and need to pushed very hard together to decrease the resistance, therefore increasing the motor speed. You don't want the little metal nub at the end of the tube that engages the arm to be mobile. Do not bend the leaf spring unless you think it has been changed, it's tension is set to not break the discs. Good luck.

On 24/10/2014 22:31, just fixed it wrote:
On Wednesday, April 22, 2009 3:17:37 PM UTC-5, wrote:
Anyone know how these old singer foot pedals work? There's not much
to this thing, but I can't for the life of me figure out how it's
supposed to work!

The one I have contains a long ceramic tube filled with a stack or
maybe 100 graphite disks. One end connects to the sewing machine
motor (I assume), and at the other end there's a contact that's
brought closer to and eventually touching the contact on the end of
the tube as the foot pedal is depressed.

The foot pedal was dropped and I'm trying to repair it, but so far I
can't get any variable speed out of it - I get either off (when the
contact isn't touching the end of the graphite-disc-tube) or ON-HIGH
when the contact touches the end of the graphite-disc-tube.

Sorry for the miserable description - it's been about a month since I
had the thing apart - just figured I'd post here to see if anyone had
any suggestions.

thanks!

-Pete

Just fixed one of these and found this ancient thread. Thought I might put this up for people finding this thread. The carbon discs get burned, so go through the stack and discard any that have broken, then take a very fine sand paper (600 grit)and lightly sand both faces of each disc. Make up the space for any discards or accidentally broken while sanding discs with some small washers. It is important that the gap be filled with something strudy, not foil or steel wool. We don't want springinees to the stack. The discs are poor conductors and need to pushed very hard together to decrease the resistance, therefore increasing the motor speed. You don't want the little metal nub at the end of the tube that engages the arm to be mobile. Do not bend the leaf spring unless you think it has been changed, it's tension is set to not break the discs. Good luck.


Good on you.

I went to an open day with these people
http://www.tfsr.org/
they have a room , with expert retirees , repairing hand and powered
sewing machines as well as all the carpentry and engineering tool
repair/reconditioning

On 25/10/2014 14:09, N_Cook wrote:
On 24/10/2014 22:31, just fixed it wrote:
On Wednesday, April 22, 2009 3:17:37 PM UTC-5, wrote:
Anyone know how these old singer foot pedals work? There's not much
to this thing, but I can't for the life of me figure out how it's
supposed to work!

The one I have contains a long ceramic tube filled with a stack or
maybe 100 graphite disks. One end connects to the sewing machine
motor (I assume), and at the other end there's a contact that's
brought closer to and eventually touching the contact on the end of
the tube as the foot pedal is depressed.

The foot pedal was dropped and I'm trying to repair it, but so far I
can't get any variable speed out of it - I get either off (when the
contact isn't touching the end of the graphite-disc-tube) or ON-HIGH
when the contact touches the end of the graphite-disc-tube.

Sorry for the miserable description - it's been about a month since I
had the thing apart - just figured I'd post here to see if anyone had
any suggestions.

thanks!

-Pete

Just fixed one of these and found this ancient thread. Thought I might
put this up for people finding this thread. The carbon discs get
burned, so go through the stack and discard any that have broken, then
take a very fine sand paper (600 grit)and lightly sand both faces of
each disc. Make up the space for any discards or accidentally broken
while sanding discs with some small washers. It is important that the
gap be filled with something strudy, not foil or steel wool. We don't
want springinees to the stack. The discs are poor conductors and need
to pushed very hard together to decrease the resistance, therefore
increasing the motor speed. You don't want the little metal nub at the
end of the tube that engages the arm to be mobile. Do not bend the
leaf spring unless you think it has been changed, it's tension is set
to not break the discs. Good luck.


Good on you.

I went to an open day with these people
http://www.tfsr.org/
they have a room , with expert retirees , repairing hand and powered
sewing machines as well as all the carpentry and engineering tool
repair/reconditioning


just noticed this resource on their site
http://www.tfsr.org/publications/tec...achine_manual/

On Fri, 24 Oct 2014 14:31:21 -0700 (PDT), just fixed it
wrote:


Just fixed one of these and found this ancient thread. Thought I might put =
this up for people finding this thread. The carbon discs get burned, so go =
through the stack and discard any that have broken, then take a very fine s=
and paper (600 grit)and lightly sand both faces of each disc. Make up the s=
pace for any discards or accidentally broken while sanding discs with some =
small washers. It is important that the gap be filled with something strudy=
, not foil or steel wool. We don't want springinees to the stack. The discs=
are poor conductors and need to pushed very hard together to decrease the =
resistance, therefore increasing the motor speed. You don't want the little=
metal nub at the end of the tube that engages the arm to be mobile. Do not=
bend the leaf spring unless you think it has been changed, it's tension is=
set to not break the discs. Good luck.

Very interesting. I started with my mother's treadle machine, and I
have a barely "portable" from about 1950, but I mostly use a White
rotary that 35 years ago my girlfriend's roommate found on the sidewalk
being thrown away in NYC and managed to squeeze into the back of her VW
bug, even with the table. She was gay and said that I was the only guy
she liked at all.

But I'm posting because, in my search for zig-zag, I came across a
Singer that depended on vacuum to control the speed. The person who
gave it to me had lost the foot pedal, but I could test it by putting a
hose on the connector and sucking on it. The harder I sucked the faster
it went!!! Eventually I found a used pedal that wasn't too much money.

I wonder if that uses the same carbon discs. I sort of doubt it, but to
find out I'd have to take apart the machine, not just the pedal.

On Wednesday, April 22, 2009 4:17:37 PM UTC-4, wrote:
Anyone know how these old singer foot pedals work? There's not much
to this thing, but I can't for the life of me figure out how it's
supposed to work!

The one I have contains a long ceramic tube filled with a stack or
maybe 100 graphite disks. One end connects to the sewing machine
motor (I assume), and at the other end there's a contact that's
brought closer to and eventually touching the contact on the end of
the tube as the foot pedal is depressed.

The foot pedal was dropped and I'm trying to repair it, but so far I
can't get any variable speed out of it - I get either off (when the
contact isn't touching the end of the graphite-disc-tube) or ON-HIGH
when the contact touches the end of the graphite-disc-tube.

Sorry for the miserable description - it's been about a month since I
had the thing apart - just figured I'd post here to see if anyone had
any suggestions.

thanks!

-Pete

My wife has a 35 year old Singer with the same type of speed control. Hers developed the same problem several years ago and increasing the disc tension slightly fixed the problem. If yours is unserviceable how about an old time sewing machine store? We have a guy around here that's been in the vacuum cleaner business since Columbus came over. He has a supply of old vacuums from day one. Perhaps you might be able to find someone like that who may have one of these just collecting dust. I would have to agree with Jeff about not using a PWM type of speed control. For the same reason I never put one on my 1953 Lionel train set. Lenny

On 25/10/2014 8:31 AM, just fixed it wrote:
On Wednesday, April 22, 2009 3:17:37 PM UTC-5,
wrote:
Anyone know how these old singer foot pedals work? There's not
much to this thing, but I can't for the life of me figure out how
it's supposed to work!

The one I have contains a long ceramic tube filled with a stack or
maybe 100 graphite disks. One end connects to the sewing machine
motor (I assume), and at the other end there's a contact that's
brought closer to and eventually touching the contact on the end
of the tube as the foot pedal is depressed.

The foot pedal was dropped and I'm trying to repair it, but so far
I can't get any variable speed out of it - I get either off (when
the contact isn't touching the end of the graphite-disc-tube) or
ON-HIGH when the contact touches the end of the
graphite-disc-tube.

Sorry for the miserable description - it's been about a month since
I had the thing apart - just figured I'd post here to see if anyone
had any suggestions.

thanks!

-Pete

Just fixed one of these and found this ancient thread. Thought I
might put this up for people finding this thread. The carbon discs
get burned, so go through the stack and discard any that have broken,
then take a very fine sand paper (600 grit)and lightly sand both
faces of each disc. Make up the space for any discards or
accidentally broken while sanding discs with some small washers. It
is important that the gap be filled with something strudy, not foil
or steel wool. We don't want springinees to the stack. The discs are
poor conductors and need to pushed very hard together to decrease the
resistance, therefore increasing the motor speed. You don't want the
little metal nub at the end of the tube that engages the arm to be
mobile. Do not bend the leaf spring unless you think it has been
changed, it's tension is set to not break the discs. Good luck.


**Yep. My SWMBO thinks I am a hero. I fixed her foot control more than a
decade ago, using your method. It still works fine. More recently, part
of the system which feeds the cloth was dodgy. I stipped it down and
fixed that too. Bugger me, those damned things are complicated.


--
Trevor Wilson www.rageaudio.com.au

On 09/11/2014 03:52, Trevor Wilson wrote:
On 25/10/2014 8:31 AM, just fixed it wrote:
On Wednesday, April 22, 2009 3:17:37 PM UTC-5,
wrote:
Anyone know how these old singer foot pedals work? There's not
much to this thing, but I can't for the life of me figure out how
it's supposed to work!

The one I have contains a long ceramic tube filled with a stack or
maybe 100 graphite disks. One end connects to the sewing machine
motor (I assume), and at the other end there's a contact that's
brought closer to and eventually touching the contact on the end
of the tube as the foot pedal is depressed.

The foot pedal was dropped and I'm trying to repair it, but so far
I can't get any variable speed out of it - I get either off (when
the contact isn't touching the end of the graphite-disc-tube) or
ON-HIGH when the contact touches the end of the
graphite-disc-tube.

Sorry for the miserable description - it's been about a month since
I had the thing apart - just figured I'd post here to see if anyone
had any suggestions.

thanks!

-Pete

Just fixed one of these and found this ancient thread. Thought I
might put this up for people finding this thread. The carbon discs
get burned, so go through the stack and discard any that have broken,
then take a very fine sand paper (600 grit)and lightly sand both
faces of each disc. Make up the space for any discards or
accidentally broken while sanding discs with some small washers. It
is important that the gap be filled with something strudy, not foil
or steel wool. We don't want springinees to the stack. The discs are
poor conductors and need to pushed very hard together to decrease the
resistance, therefore increasing the motor speed. You don't want the
little metal nub at the end of the tube that engages the arm to be
mobile. Do not bend the leaf spring unless you think it has been
changed, it's tension is set to not break the discs. Good luck.


**Yep. My SWMBO thinks I am a hero. I fixed her foot control more than a
decade ago, using your method. It still works fine. More recently, part
of the system which feeds the cloth was dodgy. I stipped it down and
fixed that too. Bugger me, those damned things are complicated.



Is the resistance variation due to surface effects rather than bulk effect.
I dug out a couple of ancient boxes , 1930s?,of some locally made
"Health Ray Infra-Red Carbons" from when you could say on the box that
such rays cured everything except cancer and no namby-pamby H&S concerns
about covering the heated rods to stop toasting forks or whatever.
Anyway (before mm days but) about 8mm diameter and 150 mm long.
The minimum resistance over that length about 0.7 ohms but is highly
critical of how pointy and pressured the DVM probe tips are.