An alarm is activated when when the barbed wire around my house is cut.

[IanB]

Radio shack and ebay have several 12v relays.  How do I find which one is the normally closed relay? Is there some specific name to it?

Normally closed will usually be marked as "NC". However, many relays will have three terminals and can be wired as normally open or normally closed depending on which pair of terminals you choose.

[Kjelt]

@da1944 see IanB reply. Can you use a multimeter and do you have one? Would be a great help for you to test some things if you have problems, you don't need a very expensive one.

[vk6zgo]

I built a 50 ft. barbed wire fence at the back of my house.  I would like to connect the barbed wire to a simple alarm system that emits sound when the barbed wire is cut.  Please help.

Do you live in that big greystone house with the towers?

[AlfBaz]

Do you live in that big greystone house with the towers?

http://youtu.be/JaLjwSpZ6Cs

[vk6zgo]

I was thinking more along the lines of this place:
http://images.smh.com.au/2011/09/01/2595564/pentridge-729-420x0.jpg

[G7PSK]

If 1st and 2nd graders use wire cutters what do 3rd graders use, Bangalore torpedoes. Where is this school sounds more like a reform centre.

[da1944]

The place is United States of America.  They start bringing guns.

[da1944]

Thanks Kjelt.  I do have a multimeter. I can measure voltages and resistances.  Ian B's message is very useful.  It should help me find a normally closed relay.  I have placed some orders at eBay to buy the components.
I do not feel comfortable in my ability to build an electronic circuit.  Is there somewhere I can buy a commercially made circuit.  I would be glad to pay someone who is able to make such a circuit for me.  I have several ac to dc convertors for power supply. 
Thank you for your help.

[Kjelt]

Are you sure you don't want to start with this hobby, it can be very addictive 
If you don't want to start and need help, better find someone more experienced closer in your vicinity, an free add in your local supermarket or contact the local radio ham amateurs is an idea perhaps?

[tautech]

Thanks Kjelt.  I do have a multimeter. I can measure voltages and resistances.  Ian B's message is very useful.  It should help me find a normally closed relay.  I have placed some orders at eBay to buy the components.
I do not feel comfortable in my ability to build an electronic circuit.  Is there somewhere I can buy a commercially made circuit.  I would be glad to pay someone who is able to make such a circuit for me.  I have several ac to dc convertors for power supply. 
Thank you for your help.

How many strands of barb wire are there in the fence?
If one is cut will both ends catch on another barb wire and still complete the circuit?
You may be better with plain smooth wires?

If you choose to connect an electric stock fence unit, you would be well advised to replace the barbs with plain wires so nobody could get tangled and possibly electrocuted with the pulses.
Yes, it can happen and I have seen it with livestock....then deadstock. 

[da1944]

Thanks tautech.  Great point.  There are 2 parallel wires which are connected in series.  I will now use only one line. High voltage lines are too scary for me.
Thanks again, Kjelt.  I am already getting addicted.  I have ordered parts for 3 different circuits, but do not feel confident.  Therefore, it would be nice to have a commercially available back-up device.  I live in Brownsville, Texas....a small town located at the southeast tip of Texas.  It is not easy to find an electronics professional.

Regards