The New Zealand Railways Magazine, Volume 4, Issue 3 (July 1, 1929)
Electric train operation in itself is a big boon from the point of view of the traffic department, but not a little advantage also arises from the adoption of improved signalling equipment, which goes hand-in-hand with every conversion scheme. The signalling department seems likely, in the near future, to play an exceptionally important part in railway operation, for we are now on the brink of vast developments in signalling methods. Reference has previously been made in these columns to the utilisation in Germany of the metal selenium in connection with train signalling. Further developments in the use of selenium cells have now taken place in Germany, and, before long, this accommodating page 43 metal may be used on a large scale in train signalling.
In Bavaria a most interesting system of train control has been established, employing a mirror which, operated in association with selenium cells and an intensifying device, automatically applies the brakes to a train that has failed to pull up at a stop signal. When illuminated, selenium becomes a conductor of electricity. Making use of this property, there has been produced a many-faceted mirror of about four inches diameter, and, by placing this mirror in position on the semaphore, the light is caught from an approaching locomotive and reflected back to the engine where it strikes a selenium cell. The impulse is then communicated to a relay, furnished with an intensifier, this actuating the train brakes. The control arrangement is equally useful for lessening train speeds on passing first warning signals, or to prevent speeds above a set maximum being attained by trains passing over any particular length of track. Altogether, these developments with selenium cells promise to open up a most profitable avenue of study for the signal engineer.