# Copper cylinders/pipe for solenoid coil formers... ?

Thread Starter

#### Hamlet

Joined Jun 10, 2015
517

Forgive my lack of artistry, but as drawn, we have two cylinders, #1, & #2.

Both are made of copper or brass or some other non-ferrous metal.

Two is a perfect cylinder, but One has a slot cut along it's length.

Now, If I wrap a series of wire turns around both, individually, and power them up like a solenoid,
compare the two, does either have an advantage? (assuming I want the most magnetic force)

Now, hypotheticals aside, I am experimenting with using copper & brass pipe for DIY solenoids,
and I wanted to understand whether I was short-circuiting the magnetic field
by using a conductive cylinder as a coil form, and if
cutting a slot would be an improvement...

If I'm not being clear, please ask questions.

#### SamR

Joined Mar 19, 2019
4,811
Umm... A question if I read you correctly... Is copper magnetitic?

#### DeadShort2020

Joined Jun 4, 2020
5
View attachment 225055
Forgive my lack of artistry, but as drawn, we have two cylinders, #1, & #2.

Both are made of copper or brass or some other non-ferrous metal.

Two is a perfect cylinder, but One has a slot cut along it's length.

Now, If I wrap a series of wire turns around both, individually, and power them up like a solenoid,
compare the two, does either have an advantage? (assuming I want the most magnetic force)

Now, hypotheticals aside, I am experimenting with using copper & brass pipe for DIY solenoids,
and I wanted to understand whether I was short-circuiting the magnetic field
by using a conductive cylinder as a coil form, and if
cutting a slot would be an improvement...

If I'm not being clear, please ask questions.
As long as the current in your solenoid is not changing, the conductive former makes no difference.
When you do change the current, a conductive former will resist changes in the resulting magnetic field. It acts as a shorted turn. The former with the slot will have much less effect.

The result is that the magnetic field changes more slowly. If your solenoid is pulling on some iron, the force on it will change more slowly with the conductive former.

The amount of this effect depends on the conductivity and geometry of the conductive former. If your former is made from a superconductor, the change in the magnetic field inside the former may take forever.

The result may be either good or bad, depending on your application.

Ted

Thread Starter

#### Hamlet

Joined Jun 10, 2015
517
The result is that the magnetic field changes more slowly. If your solenoid is pulling on some iron, the force on it will change more slowly with the conductive former.

The amount of this effect depends on the conductivity and geometry of the conductive former. If your former is made from a superconductor, the change in the magnetic field inside the former may take forever.

The result may be either good or bad, depending on your application.

Ted

Great! Now I understand. Nice addition about the superconductor. That will come in handy any day now... hopefully!

#### jpanhalt

Joined Jan 18, 2008
11,087
Remember, the iron core of your solenoid will be moving inside the coil and whatever it is formed on. There you might see an effect. That is easy and fun to demonstrate with a small neodymium magnet and aluminum tube (say an arrow shaft). When dropped through the shaft it will fall considerably slower than in air outside the tube or in a non-conductive tube. Copper does the same.

Here's a short discussion: https://physics.stackexchange.com/questions/10827/is-aluminium-magnetic/10832
And another demonstration (Lenz effect): https://terpconnect.umd.edu/~wbreslyn/magnets/is-aluminium-magnetic.html

I would suggest a non-conductor former such as thin bakelite or rigid plastic tube, Even hardened paper.

#### DickCappels

Joined Aug 21, 2008
10,067
Lenz's law states that the conductor will generate a counter-magnetic field in the opposite direction of the change in the magnetic field, which in this case I assume that the windings will be around the circumference of the tube, therefore the generated field will be running perpendicular to the winding, along the length of the tube. A magnetic change in magnetic field along this axis would generate current that in turn provides the counter magnetic field spoken of in Lenz's law. This current will primarily be around the perimeter of the tube, and would be stopped by the slot in coil One. Check me on this.

#### shortbus

Joined Sep 30, 2009
9,938
Why do you guys think the coil former has to be nonmagnetic? An ASCO Redhat solenoid valves coil has a steel/iron cylinder wrapped in wire, and they are and have been for many years, the "go to" solenoid valve in industry. So I'd say that the material of the former is immaterial to its working.

Thread Starter

#### Hamlet

Joined Jun 10, 2015
517
Really good replies, folks, thank you! I'm not a Maxwell or Faraday, so I might not be able to understand
this solenoid subject as much as I might like. On the other hand, (right hand?) we can get a little closer
to an understanding if we work our brains, eh?

I am primarily concerned with D.C. solenoids. However, it occurs to me that A.C. solenoids/contactors sometimes have laminated plungers. As these eddy currents cause heating, that would be a good thing.

jpanhalt makes a good point about Lenz effect. If the tube is a non-ferrous metal, it will experience Lenz interaction, as when the coil is energized, it will impart magnatism into the plunger, and a moving magnet
in a conductive tube will experience an oposing reactive force.

DickCappels cites what I believe is the right-hand rule, with the Lenz force being perpendicular to the direction
that the plunger travels. Cutting a slot would "open" the circuit.

shortbus shares his experience, which is valuable. I have not tried building a steel/steel solenoid, but now I would like to try. I imagine a construction with silicon steel would be optimal, for less residual magnetism, but I'll use whatever I have on hand. I assumed an all steel construction would be a non-starter, but now I can see the advantage in the opportunity for a much stronger magnetic circuit. I seem to recall an automotive starting
solenoid/relay having an all steel construction. Of course, ruggedness might win the day over efficiency in such designs.

Finally, I am yet unclear on the issue of eddy-currents. I understand how, in a transformer, the reversing current can lead to heating and inefficiency... The current for a power transformer flips back-and-forth 60 times a second, (sheesh, what's a magnetic flux to do with itself?) What I am uncertain about is what happens with D.C., or if you mind, the first 1/2 of the A.C. cycle? As the current rushes into the coil, rises to equilibrium, and a corresponding rise in flux,

does there occur ANY iron (or copper or aluminum or steel) losses, or heating, with 1/2 a circulation of induced current in a solid core or tube or plunger?

Or, is this ONLY a problem upon the REVERSAL of current?