Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Power supply, low-voltage ('Westminster pattern' very-low-voltage supplies are best)

Power supplies for Electromagnetism

When a wire or coil of thick wire is connected across the terminals of a low-voltage power supply it effectively provides a short circuit. 
 
Most general purpose power supplies are designed so that, under these conditions, an output circuit-breaker operates to switch off the unit. This protects the transformer and rectifiers from damage due to overheating. 
 
The best solution to this problem is to have a special, very-low-voltage power supply for electromagnetism (the Westminster pattern). This has a special transformer with a few, very thick turns as its secondary. The maximum AC voltage is 2 V rms, with a 1 V centre tap. High current rectifier diodes are used to give full-wave rectified DC output at 1 V.

When any of these outputs are shorted, the current flow in the transformer secondary is about 8 A. Most of the power is dissipated in the external circuit, so that the transformer is undamaged. However, the external circuit will become hot. 
 
If a general-purpose power supply is used, even when set to 1 or 2 V AC or DC, the heat generated in the transformer may damage it or, at best, cause a fuse to operate which can only be replaced by an expert.