THE WAY TO GO COMMANDO

Areal advantage of electrical and instrument work, especially in winter, is that it can be done on the kitchen table. Here there is an abundance of warmth and light, with only minor grumblings from the distaff side. So, it was on a particularly wintry winter’s eve that I sat at our kitchen table contemplating my Norton’s clocks.

Magnetic speedometers are relatively simple compared to the earlier Chronometric types, with their clock-like escapement mechanism. In a magnetic speedometer, the rotating cable spins a magnet separated by a small air gap from an aluminium disk mounted on a spindle. The magnet induces eddy currents in the disk. Electric currents flowing in a conductor in a rotating magnetic field create an angular force such that the disk tries to catch up with the magnet. The faster the magnet turns, the further the disk rotates against its restraining hairspring. Attach a pointer and scale and you have a simple but effective speedo or tacho, one that an adventurous soul might attempt to repair.

Referring to Graham Blighe’s book ‘Magnetic Speedo Repair’, I took the speedo mechanism out of its case and immediately spotted a problem, possibly ‘the’ problem. The face plate was loose and obstructing the pointer. A quick and easy repair perhaps? I removed the pointer and face plate and then replaced the pointer. Another test with my drill showed the pointer was moving smoothly and in proportion to the rotation speed.

I would need to check the calibration but the speedo was now working. All it needed was a new bezel, seal and glass, and screws to replace the crumbling plastic rivets that had failed