Deflection magnetometer consists of a small magnetic needle pivoted on a sharp support such that it is free to rotate in a horizontal plane. A light, thin, long aluminium pointer is fixed perpendicular to the magnetic needle. The pointer also rotates along with the needle (Figure).
There is a circular scale divided into four quadrants and each quadrant is graduated from 00 to 900. A plane mirror fixed below the scale ensures, reading without parallax error, as the image of the pointer is made to coincide exactly with pointer itself. The needle, aluminium pointer and the scale are enclosed in a box with a glass top. There are two arms graduated in centimetre and their zeroes coincide at the centre of the magnetic needle.