1 - Dip compass

The Chasselon dip compass measures the dip I of the Earth magnetic field.

A dip compass designed and made by Brunner was used at Cape Horn in 1882-1883 during the first International Polar Year. The compass on display was used by J.P. Rothé in Scoresby Sund (Greenland) in 1932-1933 during the second International Polar Year. The English and French designers (Cambridge Instrument for the Kew Dip Circle model, and in France the Brunner - Chasselon workshops) commercialised dip compasses as late as 1950.

The dip compass has a magnetic needle. The needle moves around a horizontal rotation axis, going through its centre of gravity and perpendicular to its magnetic moment, which can be read from the line of the needle's point. If the rotation axis is perpendicular to the magnetic meridian, the needle's moment is in line with the direction of the Earth magnetic field F. The extremities of the needle's rotation axe roll on two agate sheets situated on the same horizontal plane. To measure the dip, the user finds the position of the needle's extremities on the apparatus's graduated vertical circle. The vertical circle is considered normal at the needle's rotation axis. The graduation line 0°-180° is made horizontal when the apparatus is levelled.

The main drawback with this type of instrument is the impossibility of obtaining a perfectly cylindrical pivot axis, with the result that the needle tends to have privileged equilibrium positions. Two instruments in the same time and place can give values of I, which differ by as much as two arc minutes.