A 50 MPH Isochronous speedometer, by B. Bonniksen of Coventr...

Hammer

£480

A 50 MPH Isochronous speedometer, by B. Bonniksen of Coventry, circa 1912, patent 2052/12, dial stamped 721, milled edge and bevelled glass, diameter 8 cm.
The isochronous speedometer was first patented in 1912 by its inventor Bahne Bonniksen. Bonniksen (1835-1935) was a Danish horologist who was a watchmaker of considerable talent and is most famous for having invented the 'Karussel' mechanism in 1892 which revolutionised watch movements. When the isochronous speedometer became available in 1912, it likewise revolutionised the accuracy of the speedometer due the clock-type movement at it's heart.
Before 1912 speedometer mechanisms relied mostly on 'governor' type systems where spinning bob-weights would be flung further outwards as the road speed increased. These devices were rather inaccurate, being affected by vibration, friction and wear in the moving parts. Bonniksen's new speedo used a totally fresh approach. He reasoned that speed is simply distance divided by time. His mechanism measures time using a watch-type movement - it has a balance wheel and escapement familiar to anyone who has taken apart a watch or small clock. And measures distance by 'counting' the number of revolutions of the speedo cable, which is proportional to road speed. Both of these factors are measurable to a high degree of accuracy. The clever bit of the movement is the way it 'samples' time and distance over fixed 2.5 or 5 second increments - and feeds the resulting speed information to the indicator hands.