How can I make a bar of iron magnetic and then not?

Answer

Best,

You can relatively easily make electromagnets from the forks of the forklift truck. You do this simply by wrapping electrical wire around the spoons. Of course you have to make sure that this winding is not crushed by the load, you can place the winding at the base of the forks, for example. The insulation of the wire can be lacquer, which is thinner and therefore more compact than plastic insulation, and is certainly sufficient for the low voltage differences.

If you want to take a metal object on (or under?) both spoons, it is best that you wrap the two spoons in the opposite direction. In this way the magnetic flux runs almost entirely through the metal object, and the force effect will be the greatest.

To turn the spoons into electromagnets, you still have to ‘energize’ the winding, as they call it. This means that you let current flow through the winding, and you can easily do this by applying a voltage. If you then put a switch in between, you can simply turn your electromagnets on and off.

To see that your current does not become too large in your winding, it is best to calculate this in advance. An excessively large current can cause problematic strong heating.
The current through the wire can be calculated using Ohm’s law:
I = U/R
with I the current in Ampere (A), U the voltage in Volt (V) and R the resistance in Ohm (Ω).
You can calculate the resistance R you need for this using Pouillet’s law:
R = ρ.l/A
with ρ the resistivity (1.67×10^-8 Ω m for copper at 20°C), l the length of the wire in meters (m), and A the cross section of the wire in square meters (m2).
If you know the current I, you can also calculate the current density j ( in amperes per square meter, A/m² ) calculate :
j = I / A
It is best not to take the current density too high, because then your winding will heat up strongly. For a current density j = 500000 A/m² (=0.5A/mm²) I think you should be fine.

If your current density is too high you can do 2 things: lower your voltage U, or add a resistor R_series in series with your winding. With the latter, the series resistance must then be added to the winding resistance in order to know the total resistance.

The greater the number of windings and the greater the current, the greater the force effect will be. The force action is a bit more difficult to calculate. For this I recommend a numerical simulation, eg David Meeker’s FEMM program, but that is already advanced material.