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Friday, April 4, 2014

Crookes' work

A Crookes Tube. Credit: Wikipedia
Crookes made a career of being a meteorologist and lecturer at multiple places. Crookes worked in both the fields of chemistry and physics. The salient characteristic of his work was the originality of the conception of his experiments, and his skill in their execution.
His work on electricity in high vacuum, leads us to cathode rays tubes (the technology behind the creation of television).The Crookes tube is an early experimental electrical discharge tube, with partial vacuum, in which cathode rays, streams of electrons, were discovered. The Crookes tube consists of a partially evacuated glass container of various shapes, with two metal electrodes, the cathode and the anode, one at either end. When a high voltage is applied between the electrodes, cathode rays (electrons) are projected in straight lines from the cathode. It was used by Crookes, Johann Hittorf, Julius Plücker, Eugen Goldstein, Heinrich Hertz, Philipp Lenard and others to discover the properties of cathode rays, culminating in J.J. Thomson's 1897 identification of cathode rays as negatively charged particles, which were later named electrons
Crookes tubes are now used only for demonstrating cathode rays.

Crookes radiometer in action. Credit: wikipedia
Another interesting apparatus is the Crookes radiometer, consists of an airtight glass bulb, containing a partial vacuum. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity. Today the device is mainly used in physics education as a demonstration of a heat engine run by light energy. 


It is still manufactured and sold as an educational aid or curiosity.
The radiometer is made from a glass bulb from which much of the air has been removed to form a partial vacuum. Inside the bulb, on a low friction spindle, is a rotor with several (usually four) vertical lightweight metal vanes spaced equally around the axis.
The vanes are:
  • polished or white on one side,
  • black on the other.
When exposed to sunlight, artificial light, or infrared radiation (even the heat of a hand nearby can be enough), the vanes turn with no apparent motive power, the dark sides retreating from the radiation source and the light sides advancing.
Cooling the radiometer causes rotation in the opposite direction.
To know more see radiation pressure.
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