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)Measuring Magnetic Field StrengthThe 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.