The above picture is of a set of coils where the mechanical workshop built the former and Neville added the wire. He also built the power supply and control circuit so that the user can vary the amount of field produced within the coil.
The Helmholtz coils (pictured above right) produce a cylinder in the middle of uniform magnetic field, within witch the user can carry out an experiment or a series of experiments.
The coils pictured above are about 30cm diameter, however there have been circular coils made up that are ¾m diameter and square ones (below) that are big enough for the user(s) to stand in. Some of the smallest coils we have built are 10-20mm diameter have been produced, however, not many jobs have needed coils that small.
This set of coils comes in 2 parts and is big enough that someone could potentially sit in the middle. Because it is so big, it is difficult to access things in the centre of the field.
The first part of this coil set is an outer ring that reduces or eliminates the earths field in the X, Y and Z direction, and the second is an inner set of coils that the user can control the orientation of the field. This style of setup is so that the user can see if the chemistry of certain molecules is affected by the orientation of the earths field, by moving the field without having to worry about the earths field in their calculations.