How to vary the current using batteries? I am making a door bell as a school assignment. It works by having a solenoid produce a magnetic field which attracts a pice of iron attached on a conductor. when the iron is attracted towards the solenoid the current is broken so it falls back, inducing the magnetic field again. This will produce a frequency. The investigation is based on the influence the current has on the frequency.
I have tried to power the circuit by ten 4.5 volt battery packs connected in parallel as well as in series. But however I connect the batteries to the circuit Current will stay constant, even when I disconect battery packs.
In series I get a constant current of 3 amps. In parallel I get a current of 7 amps maximum.
What should I do to vary the current?

 A: A perfect battery would deliver a current determined only by its load but perfect batteries do not exist.  If you short a real battery, you do not get an infinite current.  You can model the behaviour of a real battery by assuming that there is a resistor inside.  This is called its "internal resistance".  Of course, no one deliberately puts a resistor inside a battery but it behaves as if there is one.  The value depends on the type of the battery.  For a $12V$ lead acid battery (e.g. in a car), this resistance is very low and they can deliver a very high current.  8 AA batteries in series will also produce $12V$ but can only deliver much lower currents.
If your battery has an internal resistance of $1.5\Omega$ and your solenoid has a very small resistance then it would explain your problem.  If you short one then you will get $3A$.  Connect two in series and short them and you will still get only $3A$ as the resistance will double as well as the voltage.  Connect two in parallel and short them and you will get $6A$ as the resistances will be in parallel.  Draw these "internal resistors" explicitly in your circuit and it should make more sense.
If you want more current then you need a battery of a different type with a lower internal resistance but, in this case, I don't think that you want this.
A: You can use potentiometer in series with the battery to vary the current.
A: I had a similar problem and the best thing to use would be an Arduino. This is because firstly, Arduino is small - to fit in the project. Secondly, you can use resistors and program them to release certain volts of current. You can set the parameters. This worked for me!
