"Harry Bloomfield" wrote in message
...
Dave Plowman (News) brought next idea :
Quite. Although even eddy current speedos could be far more accurate than
that spec.

Then there were chronometric ones. You'd laugh at a clock which couldn't
better 10% accuracy even in the 19th century.

Yep, the calibrated speedo in traffic cars.

Do traffic cars still use those in preference to GPS? I presume nowadays if
they do measure wheel rotations, everything is electronic, with no moving
parts other than the object that rotates at (a proportion of) wheel speed,
but now sensed by a Hall effect sensor and then rate of pulses (speed)
determined electronically - so there's no spring in the gauge to need to be
recalibrated as it loses its springiness. I wonder if they are recalibrated
periodically as a car's tyres wear down.

I remember in the 1970s we were staying on a caravan site and the guy in the
next caravan was a traffic policeman who described how they trained their
pursuit drivers to drive at speed - on "live" roads with real traffic, with
a "hare" driver (inevitably, in a Jag Mark II or S-type) that that "hound"
had to follow. He said that their "best hare" was a tiny lass whose feet
barely reached the pedals and who looked as if a strong wind would blow her
away, but who was almost impossible for even the best pursuit drivers to
keep up with as she weaved through traffic. Wouldn't be allowed nowadays.
H&S and all that. Reminds me of that cracking episode "Stoppo Driver" of The
Sweeney.