Fire Control Instrument

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A Fire Control Instrument, as the term is used in the Royal Navy of this period, is a mechanical or electro-mechanical communication device that helps coordinate gunnery. The interesting examples include data transmitters and receivers for communicating ranges, deflections and orders, but it extends to the simple gongs and bells used for coordinating salvo fire or the cessation of fire.

Oddly, the term was not often used to refer to items such as directors, Dreyer Fire Control Tables, etc.

Data Transmitters and Receivers

While many would grasp that there were small telephone exchanges and wireless telegraphy stations on many vessels, few readers of history would grasp that fighting ships of the early 20th century were also floating data networks. The data sent over these networks was not as flexible at the kinds that can pass over a modern digital network, but was very application specific information (ranges, deflections, orders, bearings, etc). There were 2 general means of communicating this data electrically, and thus most of the instruments on the network fell into one of two types.

Step-by-Step Instruments

Step-by-Step systems connected a transmitter to a receiver by simple wiring that conveyed changes in the transmitter's setting to the receiver, where a stepper motor would cause the receiver's configuration to mimic that at the transmitter. In World War II, these systems were called "M Type" systems, but this term seems to have come into use between the wars.

In a step-by-step network, the information transmitted over the wire was not the data value itself, but incremental changes to apply to a variable's present value as stored at each end of the transmission line. For instance, most of the Royal Navy's range transmitters and receivers were step-by-step instruments using a step-size of 25 yards. Each recorded a range for easy display in a digital display similar to an old pinball scoreboard or an odometer, but rather than having digits 0-9 on the rightmost rotor, there would be four 2 digit combinations: 00, 25, 50, 75 and the two or three rotors to the left would be single digits. When the transmitter's handle was rotated far enough to cause the rightmost rotor to change position, a commutator transmitted a pulse along the wire to a stepper motor in the receiver that would cause its rightmost rotor to also click a position. In this way, barring failures, alterations in the transmitter's indication were faithfully copied to the receiver.

Failures could, however, occur if the transition between positions was too rapid or if a power transient caused the receiver's motor to click off too many or too few positions. These errors were called missteps and would cause the transmitter and receiver to diverge. They were uncommon in a well-maintained system,[Citation needed] but were difficult to detect, and required a time-consuming recovery process.

Direct Working Instruments

Direct working systems were ones in which any change in indicated value could be instantly (or very promptly) matched by the receiver. The systems could only really go out of operation if some portion were physically damaged or disconnected, and such failures were usually apparent. Moreover, the connections of transmitters to receivers could even be redrawn in action without requiring a process of synchronising the terminals (as was needed with step-by-step systems).

The primary drawback to this sort of design was the large number of wires needed to convey the data from one end to the other.


Although different services tried different systems well before the war, the advantages offered on paper by the direct working designs were outweighed by the large number of conductors required. The Royal Navy voted strongly in favor of step-by-step systems. deploying them widely and standardising the core components of commutators and stepper motors at each end.

By World War II, advances in technology reversed this trend, particularly in the US Navy.[Citation needed]

See Also



  • Schleihauf, Bill (2001). "The Dumaresq and the Dreyer, Part I-III." Warship International 38 (1-3).
  • Admiralty, Gunnery Branch (1918). Handbook of Captain F. C. Dreyer's Fire Control Tables, 1918. C.B. 1456. Copy No. 10 at Admiralty Library, Portsmouth, United Kingdom.
  • Admiralty, Gunnery Branch (1910). Handbook for Fire Control Instruments, 1909. Copy No. 173 is Ja 345a at Admiralty Library, Portsmouth, United Kingdom.
  • Admiralty, Gunnery Branch (1914). Handbook for Fire Control Instruments, 1914. G. 01627/14. C.B. 1030. Copy 1235 at The National Archives. ADM 186/191.