Wheatstone Polar Clock, 1848-1860.
© Science Museum / Science & Society Picture Library
Wheatsone's Polar Clock works on the same principle as a polariscope, a device for measuring or demonstrating the polarisation of light. Designed by Charles Wheatstone (1802-1875), the famous English physicist, it was intended as substitute for the sundial. Using the fact that the Sun's position in the sky is always at right angles to the polarisation plane of sunlight, the device measures the horizontal direction of this plane. Scales indicating both the altitude and direction of the instrument give an indication of the time. Although les accurate than a sundial, it had the advantage that it could be used on a cloudy day or when the Sun was just below the horizon.