Hi John,
I posted your query on the LNER forum and had the following from Mick Nicholson, who knows most of what there is to know about signalling:
Very simply the bottom part of the motor casing contains the actual motor, the top part contains the "Holding Off" coil. Basically the armature turns to operate the signal and then via internal contacts cuts itself off, the "Holding Off" coil being energised then holds the mechanism "Off". Like all railway equipment the entire outfit is "Fail Safe", either a power supply failure or the signal down wire breaking will instantly cause the signal arm to go to danger. Possibly a very crude and basic explanation but practically all the books in my collection deal with mechanical signalling. Given time and asking the right people I'm sure I could come up with more. Mick Nicholson.
I posted your query on the LNER forum and had the following from Mick Nicholson, who knows most of what there is to know about signalling:
Very simply the bottom part of the motor casing contains the actual motor, the top part contains the "Holding Off" coil. Basically the armature turns to operate the signal and then via internal contacts cuts itself off, the "Holding Off" coil being energised then holds the mechanism "Off". Like all railway equipment the entire outfit is "Fail Safe", either a power supply failure or the signal down wire breaking will instantly cause the signal arm to go to danger. Possibly a very crude and basic explanation but practically all the books in my collection deal with mechanical signalling. Given time and asking the right people I'm sure I could come up with more. Mick Nicholson.