a matrix of magnets

Project summary
Although I'm eagerly learning about electronics, I still hit some walls.
I want to make a sound visualiser using 9 electromagnets placed in a grid.
On top of these magnets will be a sheet of paper with iron filings which in turn will create visually pleasing patterns.
The sound will be analysed by processing.
The magnets will only have to turn on and off.

http://img155.imageshack.us/img155/2325/img1748k.jpg

My Problem
the relays i took apart to function like an electromagnet are of this partnumber : SRU-06VDC-SL-C
this is the setup so far:

• it works with the voltage/ampere i get out of the digital Arduino I/O pins, but it is not strong enough.
• I presume i need to be working with an external power supply. which is suited for my needs? (battery? dc adapter?)
• how much voltage/ampere can i use to maximize the effects of electromagnetism?
  final question is:
• what do i use to switch the electromagnets which are powered by a higher power source? (eg. transistor....?)

i'm looking for a way to connect the arduino I/O signal to power one or more of the 9 magnets in the grid.
and which components i need to do this.

I thank this forum in advace,
you'll be seeing more of me around.
I started teaching new technologies and these are problems i run across while creating these kinds of projects with them.

• how much voltage/ampere can i use to maximize the effects of electromagnetism?

The more current you put into an electromagnet the stronger it gets until you reach the saturation point of the core. After that no more magnetism is produced.

The voltage forces the current through, the bigger the voltage the faster you will be able to change on / off.

However, if you only send the current in one direction you will be left with a residual amount of magnetism when the current is off, this might be enough to spoil your effects or you might not notice.

Thank you for your replies.
I'll take this new knowledge in consideration and will post a follow up of the project.