Question:
Has anyone a good design for electro-magnets that actually works? It seems the designs in the texts are rather weak and not very effective.
Answer:
In order to have sufficient current passing through the wire, the wire must be rated to carry at least 3 amps. Wire of this thickness can cause the electromagnet to be quite bulky, so it is suggested that you use a nut and bolt of about 10-12 cm long, or a very large nail.
The strength of the magnet depends on the number of turns of wire around the nail plus the size of the current passing through it, which is dictated by the voltage applied. So you have to use lots of wire – at least 5 metres, preferably 10, for each electromagnet.
Bumping up the voltage will increase the strength of the magnet, but if you dont have lots of turns around the nail, then you are effectively just shorting out the power supply, which causes the overload switch in the power supply to trip out. No problem, just not good practice and its inconvenient.
A way to overcome the power supply being shorted out is to put a globe in the circuit. Use a Hodson light box and the students can see the varying brightness of the globe according to the voltage and the related ability of the electromagnet to pick up, for example, paper clips.