Mercury mystery solved when sparks fly
(This story appeared on page 53 of the
Jan. 24, 1998, Science News.)
By Sid Perkins
Science News
Swirling mercury around in a glass container from which the air has been removed generates light. This curious glow - first described more than 350 years ago - came to be known as barometer light, taking its name from the scientific instrument invented in 1644 by filling a glass tube with the dense liquid metal.
Scientists at the University of California, Los Angeles have now shown that barometer light is an unusual consequence of static electricity, a familiar, but still poorly understood, phenomenon.
The flashes of light are associated with the so-called stick-slip behavior of mercury as it moves over a glass surface, says physicist Seth J. Putterman. His team reports its findings in the Jan. 15 NATURE.
When mercury comes into contact with glass, large numbers of electrons hop from the liquid metal to the solid surface, which thereby acquires a negative charge. This transfer of electrons leaves the mercury with a net positive charge and results in an attraction between the two materials.
The UCLA researchers slowly spun a horizontal glass cylinder containing mercury. As the flask rotated, the positively charged liquid metal stuck to the negatively charged glass and was dragged along with it. Eventually, gravity overcame the attraction between the materials, and the mercury suddenly slipped back, Putterman says. An electric discharge lasting only a few picoseconds and a more prolonged flash of light occurred just before each slipping event.
The interval between events, as well as the size of each event, varied, but the reasons are not clear. The researchers' data shows that the two are linked - the longer the period after a particular slipping event, the larger the subsequent event is likely to be. This pattern also characterizes earthquakes, another large-scale, stick-slip phenomenon, Putterman says.
In a separate phase of the study, the scientists bolstered the static electricity explanation for mercury's behavior even further. By shining an ultraviolet light on the surface of the liquid metal, they produced a photoelectric effect that prevented the separation of charges between the two materials. Because the mercury would no longer stick to the glass as it rotated, the liquid metal remained at rest, and no barometer light was seen.
"This desktop demonstration may help us understand the transfer of charges between surfaces and the origins of friction, two everyday phenomena that no one really understands," Putterman says.
---[
Copyright 1998 by Science Service.
All rights reserved.
---[