Paul Wainwright

Bell Telephone Laboratories, Retired
Atkinson, New Hampshire, USA

The Blackburn pendulum is a common textbook exercise that is familiar to any student of physics, but as a senior in high school in 1967 I saw it as an artistic tool. I proceeded to make a number of simple time exposure photographs, looking up at a small neon bulb attached to such a pendulum.

For the past 8 years I have used a 3-meter Blackburn pendulum in my barn with an LED, and my camera is a large-format 4 x 5 inch sheet film camera. Working at night in total darkness, the camera looks straight up, and a time exposure is made as the LED traces out the pendulum’s path. Since a Blackburn pendulum approximates two harmonic oscillators, I name my images according to the musical harmonic interval they depict.

Pendulum Light Painting 165J: Major Fourth Harmonic Interval
40 x 50 cm
Archival silver gelatin photographic print from 4 x 5 inch negative

Photographic light painting of an LED attached to a Blackburn pendulum strung to produce orthogonal periods of motion in a 4-to-3 ratio (a major fourth harmonic interval). Total exposure time was 12 min 48 sec. The exposure was started in the lower right corner of the image. The camera was motionless for the first two minutes, after which the LED’s brightness was decreased and the camera was rotated slowly 90 degrees. Because the formula for the period of motion of a pendulum contains an angular dependence, the Blackburn pendulum goes in and out of resonance as the amplitudes decrease, and in this case the rotation and exposure times were selected to make the heart shape at the beginning of the exposure.