Change-over Switch

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A Change-over Switch (usually C.O.S. in British sources, but sometimes group switch for its use in switching gun groups) is a rotary electrical switch, often a large cylindrical one that multiplexes a number of inputs to any one of a variety of sets of outputs.

These were often found within a Transmitting Station, though smaller ones were also found in gun turrets and similar stations serviced by a large number of electrical connections. They routed fire control instrument signals (e.g., to connect a range transmitter to one of several range receivers) and navyphone lines.


In 1905, the Annual Report of the Torpedo School detailed the need for switches for navyphone signal routing akin to the "group switches" supporting data circuits for allowing alternate control systems.[1] I am appalled to see I did not image the pages outlining the data circuit switches!


A typical purpose for a C.O.S. would be to manage in one nexus, the many parallel connections that should sensibly be established or altered when a ship with multiple directors and/or TSes wished to switch which director or TS was to be used.

Generally, all firing, elevation, slewing and training data cables from directors to turrets would come through the C.O.S., and each of its 3-5 rotary stop positions would establish different patterns of connection to permit some flexibility as to which turret is connected to which director. The parallel path of the several connections that need to be made for this to be done successfully is intrinsically managed by the inner rotating column within the C.O.S., and this reduces the possibility of having a switch out of place (as there is only a single switch instead of many).

As an example, imagine a ship with a a forward and an aft director that was firing its guns using the forward director. A new target emerges suddenly, and it is decided that the aft group of guns should engage this target, using the aft director. Here is how it would be done:

  1. A quick check fire would be established and the forward director would freeze in place.
  2. The aft director and aft group would slew to a pre-arranged line-up bearing and elevation as an operator within the TS threw the C.O.S. from "all guns on forward director" to "fwd guns on fwd, aft guns on aft". This action would disconnect the elevation, slewing and training connections that had previously bound the aft turrets to the forward director, and establish them with the aft director.
  3. A quick verbal check would be made to ensure that the receivers in the forward guns had not jumped due to a transient as the switch was thrown. If it succeeded, the forward guns could immediately proceed under control of the forward director.
  4. The aft director and guns would undergo a lining up of pointers to synchronize themselves. Once complete, they would be able to fire upon their new target.

This is a general description. The routing of the wiring within any ship and its C.O.S.(es) bears study before one can grasp how flexible its fire control apparatus and staff could be, and how prone it might be to degraded accuracy when changing modes in this manner. For instance, with a single Dreyer table, who would supply the range and deflection information for the new aft gunnery group? How would the range and deflection data get to the sights? Did they also get handled by the C.O.S.? Did orders signals get routed by the C.O.S.? Spotting? We endeavour to supply as much information as we can in our ship (and ship class) pages to help convey the capabilities and limitations of the different ships.


  1. Annual Report of the Torpedo School, 1905. p. 73 and Plate XXIV.


  • Admiralty, Gunnery Branch (1917). The Director Firing Handbook. O.U. 6125 (late C.B. 1259). Copy No. 322 at The National Archives. ADM 186/227.
  • 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.
  • H.M.S. Vernon. Annual Report of the Torpedo School, 1905, with Appendix (Wireless Telegraphy). Copy 22 at The National Archives. ADM 189/25.