Sound-ranging comparator Mark IV, metal, glass, maker unknown, c. 1930s, 1940s.
The object of sound ranging is to direct effective fire on a source of explosive sound in the enemy lines. This source is normally a hostile gun troop which should be neutralised immediately by a heavy concentration of fire or registered for future neutralisation.
Prior to using the comparator the sound ranging organization would have obtained records showing an approximate location of the area of enemy activity, as well as having knowledge of shell bursts, fired by a co-operating gun. A line of microphones is used to record the sound of a shell blast, and observations would be based on the comparator, which is adjusted to find 'line of best fit' from which an increment in range over or under a line left or right can be read.
A number of sliding rulers are adjustable by hand, seen through a clear glass plate, which turns. The comparator also has a diameter cursor. An adjustable scale is read through the glass. A mechanism on the side of the assembly adjusts the scales together.
The gun-to-target line is plotted and the angles subtended at the enemy gun between
the various microphones and the gun-to-target line is measured.
This is a Mark IV, 1941 vintage, noting that the Artillery Museum at North Head of Sydney Harbour also has a Mark IV of this instrument.
The instrument is marked OS 841 GA.