Radio direction finder or sentinel

This radio direction finder is built into a wooden cabinet with glass door. At the upper side of the cabinet is a button with a pointer which is rotatable over a scale of 360 degrees.
Technical information:
The button is via a shaft connected to a search coil in the cabinet. In the case are two sets of mutually perpendicular wire windings which surround the rotatable search coil as a cage. The wire coils serve as a reception window. They have a height of 26 cm and an average width of 18.5 cm. Operation: According to the drawings, the thread windings have to form a tuned circuit, through interposition with a variable capacitor. The search coil must be connected to a crystal.
Use:
During World War I Idzerda develops this direction finder or level gauche in request of military authorities. This device can detect active spy stations in the Netherlands. German zeppelins can also be traced when they fly at night over the Netherlands to return home after they bombed London.