Monday, March 28, 2011

The Crookes Railway Tube

PV Scientific's founder and master instrument maker, Jim Hardesty, in his laboratory 
with a beautiful old Crookes railway tube, also known as a paddlewheel tube.
Photo credit: Bryan T. Root, Motherlode Pictures.

During the 1870s, the British physicist, Sir William Crookes, performed a number of experiments in which cathode rays seemed to cause the movement of objects suspended in evacuated tubes. In the late 1870s, he developed a tube that provided the most spectacular demonstration of this effect: the railway tube, also known as the paddlewheel tube. This tube contains two concave or focused cathodes, one on either end, so that the polarity of the electricity flowing through the tube can be changed back and forth, and the cathode rays can be aimed at the vanes (or paddles) of mica in a paddlewheel positioned on two glass "rails" within the tube. When the cathode beam strikes the mica vanes, the paddlewheel rolls down the track. When the polarity of the electrical energy being fed into the tube is reversed, the paddlewheel rolls in the opposite direction.

A still photo of the railway tube in operation. The cathode is seen as a purple glow at 
the left. The green rectangles in the center are the glowing vanes, or paddles, spinning as the paddlewheel rolls along the glass "track" of the tube. Photo credit: Bryan T. Root, Motherlode Pictures.

Crookes was certain that the spinning effect of the wheel in the tube was caused by transfer of momentum from the impact of the corpuscles (particles) of the cathode rays, and the railway tube demonstration provided very firm support for the corpuscular theory of cathode rays. However, in 1903, some six years after J. J. Thompson discovered the electron, he wrote about the working of the Crookes railway tube in his famous book, The Discharge of Electricity through Gasses, claiming that the push of electrons alone could not explain the speed of the spinning wheel in the tube, and Thompson offered the idea that the heat generated by the electrons striking the mica paddles expanded the atmosphere on the side of the paddles being struck, thus pushing the paddles forward. Thompson's explanation is also used to describe the action of another invention of Sir William Crookes, the radiometer.

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