Homemade electromagnet, considerations - strength

[burntoastt]

I'm making a solenoid electromagnet for gcse demonstration purposes and I'm trying to get a strong, clear magnetic effect. I'v wrapped about 100 turns of enamel plated copper wire around a 4" bolt and also a 4" ferrite rod, and am applying 9v DC from a pp3 battery cell.
In both cases I got a barely noticeable magnetic field of attraction when tested with a compass, not satisfactory.

Any ideas on how i can most simply improve upon this? e.g Should i add more coils? If so how many? And should i increase the voltage?
Also does it matter if the bolt is galvanised?
The ferrite rod was from an old radio, is that ok? and is it better than an iron rod? e.g a bolt.

 

[Charles Link]

These 9 volt batteries sometimes have an internal resistance that is quite high and might have severely limited the current. Suggestion is to try again with a 1.5 Volt D cell, and if you don't get the strong magnetic field, then it is back to the drawing board. $\\$ Additional note: the D cell has a rather low internal resistance, and the solenoid very nearly short circuits the D cell. If you don't add a small series resistance to the D cell, you may get very limited use out of the D cell before you drain it. $\\$ One way to test your bolt material for magnetic properties is to bring a permanent magnet up to it. If there is a strong attraction, the bolt should be a suitable material. $\\$ A google of the internal resistance of the 9 volt battery shows that it can be anywhere from 1 to 2 ohms (for alkaline) to 35 ohms for other types. One possibility is that the battery got drained significantly in the nearly short-circuited mode before you got the compass in place to check for magnetic field strength.

 

[sophiecentaur]

Any ideas on how i can most simply improve upon this?

You are probably following the recommendation in a very basic magazine or book and you can do much better than you have done so far if you can improve on the set up.
Early magnets were horseshoe shaped - for a good reason. A C shaped iron or steel former will work much better because of the strong field between the poles. Cut up an old transformer if you want a good source of iron in the right shape.

These 9 volt batteries sometimes have an internal resistance that is quite high

Agree. You need low volts and plenty of current capacity such as you get from one or two C or D cells in series. Get a good battery holder with clean contacts and solder joints or well screwed down terminals. Anything to reduce the series resistance. You haven't mentioned the gauge of the wire but probably the more the better unless it's pretty thin (i.e. significant coil Resistance ).
If you have access to a Multimeter then you could measure the resistance and also see how much the battery volts dip when the coil is connected.
PS A ferrite rod won't give you much joy. They use ferrite because it works at high frequency and not because of its high permeability.

 

[burntoastt]

You are probably following the recommendation in a very basic magazine or book and you can do much better than you have done so far if you can improve on the set up.
Early magnets were horseshoe shaped - for a good reason. A C shaped iron or steel former will work much better because of the strong field between the poles. Cut up an old transformer if you want a good source of iron in the right shape.

Agree. You need low volts and plenty of current capacity such as you get from one or two C or D cells in series. Get a good battery holder with clean contacts and solder joints or well screwed down terminals. Anything to reduce the series resistance. You haven't mentioned the gauge of the wire but probably the more the better unless it's pretty thin (i.e. significant coil Resistance ).
If you have access to a Multimeter then you could measure the resistance and also see how much the battery volts dip when the coil is connected.
PS A ferrite rod won't give you much joy. They use ferrite because it works at high frequency and not because of its high permeability.

These 9 volt batteries sometimes have an internal resistance that is quite high and might have severely limited the current. Suggestion is to try again with a 1.5 Volt D cell, and if you don't get the strong magnetic field, then it is back to the drawing board. $\\$ Additional note: the D cell has a rather low internal resistance, and the solenoid very nearly short circuits the D cell. If you don't add a small series resistance to the D cell, you may get very limited use out of the D cell before you drain it. $\\$ One way to test your bolt material for magnetic properties is to bring a permanent magnet up to it. If there is a strong attraction, the bolt should be a suitable material. $\\$ A google of the internal resistance of the 9 volt battery shows that it can be anywhere from 1 to 2 ohms (for alkaline) to 35 ohms for other types. One possibility is that the battery got drained significantly in the nearly short-circuited mode before you got the compass in place to check for magnetic field strength.

Thanks for these suggestions! ill change the setup shortly, and let you know how it went! Btw, can the coils overlap or does this effect the magnetic field?

 

[sophiecentaur]

Thanks for these suggestions! ill change the setup shortly, and let you know how it went! Btw, can the coils overlap or does this effect the magnetic field?

You can have many layers of wire. This would be essential if you want enough turns for a usefully strong electromagnet.

 

[burntoastt]

These 9 volt batteries sometimes have an internal resistance that is quite high and might have severely limited the current. Suggestion is to try again with a 1.5 Volt D cell, and if you don't get the strong magnetic field, then it is back to the drawing board. $\\$ Additional note: the D cell has a rather low internal resistance, and the solenoid very nearly short circuits the D cell. If you don't add a small series resistance to the D cell, you may get very limited use out of the D cell before you drain it. $\\$ One way to test your bolt material for magnetic properties is to bring a permanent magnet up to it. If there is a strong attraction, the bolt should be a suitable material. $\\$ A google of the internal resistance of the 9 volt battery shows that it can be anywhere from 1 to 2 ohms (for alkaline) to 35 ohms for other types. One possibility is that the battery got drained significantly in the nearly short-circuited mode before you got the compass in place to check for magnetic field strength.

thank you, I appreciate the suggestions! i look forward to trying them out, is increasing the amount of coils a good idea?

 

[burntoastt]

For the coils i am using enamel coated copper wire, with a thickness of 0.35mm.