Question

Helmholtz coils...why is the cail made up of many small wires? Why not use a single small wire? Why not use a single large wire with a high current?

Answer 1

A Helmholtz coil is a device for producing a region of nearly uniform magnetic field, named after the German physicist Hermann von Helmholtz. It consists of two solenoid electromagnets on the same axis. Besides creating magnetic fields, Helmholtz coils are also used in scientific apparatus to cancel external magnetic fields, such as the Earth's magnetic field.

Suppose you have 10 turns and the current flown in each coil is 1 ampere than in second case the current flown in straight wire will be 10 amperes and current flown in first case is 1 ampere.

First of all why you get more magnetic field if you have more number of turns. The current flown in a wire will generate magnetic field, the magnetic field at the center of the wire loop will be

B=μ0I2R

where R is the radius of the loop. Now if you stack the loops with current in same direction then this magnetic field will be added and you will get the nn times field. If you add these loops in series you can flow just one ampere and get nn times field. It may be noted here that, if by adding the loops you construct the solenoid you will get the magnetic field

B=μ0NI

where N is the number of turns per unit length

Now comes the low current-large number vs high current-low number mode. Both cases are used practically and it depends on your design. If you want to generate high magnetic field by adding larger numbers of turns then you have to make your wire thinner (to the size of magnet). This increases the resistance in the wire and in turn increases the heating of wire. Heating of the wire will result in removal of the insulation over it and then your magnate will fail. Hence there is a rated current over which there is always a probability of the failure of magnet. It may also be noted that the stacking of loops one over the above poses difficulty in heat removal from magnet, which will further reduce the maximum current rating of the magnet.