How to Colonize an Asteroid
How to Colonize an Asteroid
The amount of force generated by the magnetic field in the preceding experiment is very small, but we can concentrate the force by winding many turns of wire into a tight coil. The addition of a piece of soft iron to the core of the coil helps to strengthen and shape the magnetic lines of flux. This arrangement is called an "Electromagnet".
An electromagnet can be made very simply by winding about 20 turns of 22 or 24 gauge insulated wire around an iron nail. It is easier to use solid core wire for this, as it holds a shape better, and a bit of tape to hold it together also helps. Connect the opposite ends of the wire to a "C" cell battery and the nail will become a magnet which will pick up small ferrous objects, like paper clips or small finishing nails. {R.10.158} (CAUTION! You will drain the battery fairly quickly connected this way, and the wire might actually get quite hot)
The force generated by an electromagnet is called the "Magnetomotive Force" (or M.M.F.), and is expressed in a unit of measure called the "gilbert". The magnetomotive force can be calculated with the following formula;
M.M.F. = 1.257 * I * N
Where M.M.F. is the magnetomotive force in gilberts, I is the current in amperes, and N is the number of turns in the coil. The important point to this formula is that the total amount of force is dependent on the ratio of turns in the coil (N) to the amount of electrical current flowing in that coil (I).
The relationship expressed above is know as "ampere-turns", and is simply the current times the number of turns.
ampere-turns = current (in amperes) times the number of turns = I * N
One of the more useful things to know about ampere-turns is that we can operate a fairly powerful electromagnet at a lower voltage if we increase the number of turns. A coil of 10 turns at 10 amperes has exactly the same magnetomotive force as a coil of 100 turns operating at 1 ampere, since both coils have the same number of ampere-turns. {R.4.2.18}{R.42.350}{R.43.459}
Magnetic fields, like electrical charges, have "positive" and "negative" poles. In magnets, these poles are called "north" and "south", referring to the north and south magnetic poles of our planet earth.
An electromagnet has the same north and south poles as a permanent magnet, and we can determine which is the north pole by following the "left hand rule". If you wrap your left hand around a coil so that your fingers are pointing in the same direction as the electrons are flowing, your thumb will be pointing towards the "north" magnetic pole. Remember that electrons in a conductor flow from negative to positive. {R.4.2.18} {R.10.118} {R.42.351}
© 1998, Robert Lyon Richards