Animal Magnetism and the No.15

Posted on June 04, 2014 by Gary Carruthers

The Anchor Pub, Bankside, London, 4th June 2014

The No.15 is an Islamic timepiece, with much of the writing in Arabic.  In the centre, time is indicated, with seconds on the left dial.  At the top, Qibla (the direction to Mecca) is shown.  And at the bottom, you can select between indicating current time, or time of one of the daily prayers, Fajr, Dhuhr, Asr Maghrib or Isha.  (You can opt for the compass to indicate North rather than the direction to Mecca.)

Knowing Qibla requires knowledge of geographic location, which the timepiece can get from the GPS on your smartphone.  It also requires knowledge of orientation, for which it uses the magnetometer.  But we've come across an interesting problem:  The magnetic fields generated by the watch.

First, there are the fields generated by currents flowing round the circuit.  Secondly, there are electromagnetic fields generated by the Bluetooth antenna.  But these are both negligible compared to the third:  The magnetic rotors.  Each hand in our watches is connected to a gear train which ends with a rotor. (See the millimetre-sized wheel in the centre of the photo).  In most motors, the rotors are soft iron, meaning they are not permanently magnetic.  Ours, for reasons I won't go into, are hard iron, which means they are permanently magnetic, and with each pulse, the magnet rotates 180°.

Mike is fuming because I've positioned the magnetometer right next to one of these rotors.  (I didn't have much choice, as it happens.)  It's right next to the rotor that displays the direction to Mecca.  So not only does the rotor affect the magnetometer reading, but this in turn spins the rotor, and the feedback loop causes the hand to go wild like some crazy animal.  It doesn't help that we're in a cast iron Victorian wharf building, with electric trains rumbling over the roof on their way in and out of Cannon Street station.

Mike now has to work out how to calibrate that all out.  Just to be doubly accurate, we've decamped to the Anchor pub just to check that the iron beams and trains aren't distorting the magnetic field.  (It's probably not an entirely appropriate place for an Islamic timepiece to be seen, but a necessary test procedure for us.)

"We do these things not because they are easy, but because they are hard." - JFK

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