Nichols radiometer

The Nichols radiometer was a precision scientific instrument devised in the early twentieth century for the direct measurement of radiation pressure, a phenomenon predicted by classical electromagnetic theory and later reinforced by quantum concepts of light. Developed in 1901 by the American physicists Ernest Fox Nichols and Gordon Ferrie Hull, the device represented one of the first successful attempts to validate experimentally the extremely small but measurable force exerted by electromagnetic radiation upon a reflective surface. The radiometer became a pivotal instrument in the study of radiation physics and contributed significantly to the acceptance of radiation pressure as a physical reality.

Background and Development

The idea that light carries momentum and can exert pressure on matter emerged from James Clerk Maxwell’s electromagnetic theory, where electromagnetic waves were shown to possess both energy and momentum. Throughout the late nineteenth century, several scientists attempted to measure this effect, but technological limitations—particularly those relating to precision instruments and environmental control—made reliable measurements extremely difficult.
In this context, Nichols and Hull sought to design an apparatus capable of isolating the delicate radiation pressure from other interfering forces such as convection currents, thermal effects, and air molecules. Their work took place primarily at Dartmouth College and later at the Astrophysical Laboratory of the Carnegie Institution, forming an important part of experimental physics at the turn of the century.

Design and Construction of the Apparatus

The radiometer devised by Nichols and Hull was based on a finely balanced torsion system. It featured two small, circular, silvered glass mirrors mounted opposite one another and suspended by a delicate quartz fibre. The entire assembly was enclosed within a chamber whose internal air pressure could be carefully regulated. Quartz was selected for the suspension fibre because of its high elasticity, minimal creep, and excellent stability under varying temperatures.
A torsion head enabled rotation of the fibre from outside the enclosure. This head was manipulated magnetically, allowing precise adjustments without disturbing the internal environment. The enclosure itself was fitted with optical windows to permit the controlled entry of a light beam, which was alternately directed onto either mirror. Each incident beam imparted a minute pressure upon the reflective surface, resulting in a measurable deflection of the mirror system.
The deflection was observed with a mirror-and-scale arrangement, a common technique in torsion-based instruments of the period. By analysing the differential movements corresponding to illumination of one mirror and then the other, the researchers could deduce the net force attributable solely to radiation pressure.

Control of Environmental Influences

A key difficulty in measuring radiation pressure lay in eliminating or compensating for non-radiative forces. Nichols and Hull addressed this challenge through methodical experimental design. By turning the mirror assembly so that light struck its unsilvered side, they were able to observe the effects of residual air movement and thermal expansion without the influence of radiation pressure, thus enabling an assessment of background disturbances.
It was found that the influence of air resistance and convection became negligible at extremely low pressures. At air pressures of approximately a few thousandths of an atmosphere, the behaviour of the suspended mirrors was dominated by the direct effect of the incident radiation. This level of environmental control marked a significant improvement over earlier devices such as the Crookes radiometer, which operates partly on thermal transpiration effects and therefore cannot be used as a reliable instrument for measuring true radiation pressure.

Measurement of Radiant Energy

The accurate determination of radiation pressure required knowledge of the energy carried by the incident beam. Nichols and Hull achieved this by measuring the heating effect of the beam on a small blackened silver disk positioned in the path of the radiation. The disk absorbed the energy and underwent a temperature rise that could be correlated with the incident radiant power.
This method proved more reliable than the bolometer originally employed. The bolometer, though sensitive, was prone to drift and required complex compensation to maintain accuracy. The substitution of the blackened disk simplified the calibration process and enabled a more consistent determination of the radiant energy.

Experimental Findings and Precision

Using the Nichols radiometer, the experimenters succeeded in obtaining quantitative agreement between the observed radiation pressure and the values predicted from electromagnetic theory. Their recorded measurements matched theoretical expectations to within approximately 0.6 per cent, an exceptional degree of accuracy for an experiment of such delicacy at the time.
Their findings provided compelling empirical support for Maxwell’s predictions and helped establish radiation pressure as a measurable and physically significant quantity. These results also laid conceptual groundwork for later developments in photonics and the quantum theory of light, including the notion of photons carrying quantised momentum.

Relationship to the Crookes Radiometer

The Nichols radiometer is sometimes mistakenly associated with the Crookes radiometer, invented in 1873 by Sir William Crookes. However, the two devices operate on fundamentally different principles. The Crookes radiometer, which consists of vanes that rotate when exposed to light, functions primarily due to thermal transpiration effects arising from temperature gradients in rarefied gas. It does not provide a direct or accurate measurement of radiation pressure.
In contrast, the Nichols radiometer was designed explicitly to suppress thermal effects and isolate the momentum transfer of light itself. Its operation relies on reflection-induced forces measured by a torsion balance—a far more rigorous and quantitative method suited to experimental physics.

Legacy and Historical Significance

The original Nichols radiometer remains preserved at the Smithsonian Institution as a historically important scientific instrument. The apparatus represents a landmark achievement in precision measurement and early photonics research. Its results, published in The Astrophysical Journal in 1903, are considered foundational contributions to the understanding of electromagnetic radiation.
The experiment holds continued significance in the context of modern optical physics, contributing to technologies and fields such as optical trapping, laser propulsion, and radiation pressure-based sensing. Reprints of the original papers have appeared in contemporary works on photonics, underscoring the continuing relevance of Nichols and Hull’s pioneering measurements.