You will already know that the magnetic field near a current gets weaker as you move further away from the current.

The magnetic field strength is inversely proportional to distance:

The magnetic field strength is proportional to current:

Combining these relationships gives:

where the constant depends on the shape of the conductor.

Field around a straight wire:

Where:

I = current throught the wire (A)

r = distance from the conductor (m)

Field in a long straight solenoid:

Where:

n = number of turns on the coil per metre (m^-1)

I = current throught the solenoid (A)

The strength of a magnetic field can be found using a device called the Hall probe. It works on a principle based on the Hall effect.