Goniometer - The name means 'angle measurer'. In radio context, it is a set of coils arranged to determine the bearing (direction) of a transmitter. The scan at right is from the 1938 Admiralty Handbook of Wireless Telegraphy, a two-volume set which has spent more than half of its existence in my collection.
The name Goniometer was chosen for the blog because although its relevance to modern electronics is marginal, the word stands out eyegrabbingly in a page full of search results; this gets more traffic to the blog without resorting to the less ethical SEO methods.
A goniometer is useless without a set of source antennas, and at left I have another page from the Handbook, showing a set of Bellini-Tosi direction-finding antennas. The triangular loops at the top are the antennas, and the three two-turn coils at the bottom are simplified representation of the goniometer coils.
The operation of the system is straightforward. The sense coil ('S' in Fig.18) is rotated until a null (little or no output) is heard in the receiver. The sense coils are connected to a pointer, which indicates the direction of the source transmitter.