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.

Wednesday, March 9, 2011

Musschenbroek and the First Electrical Capacitor

Pieter van Musschenbroek, 1692-1761
"I wish to inform you of a new, but terrible experiment, which I advise you on no account personally to attempt." 
   ~Pieter van Musschenbroek

March 14th, 2011, marks the 319th anniversary of the birth of Pieter van Musschenbroek, the fellow usually credited with the discovery and initial investigation of the world's first electrical capacitor.

Called the Leyden jar after Holland's University of Leyden where Musschenbroek taught, this instrument was independently discovered at about the same time by Ewald Jurgen von Kleist, a Pomeranian cleric.

Musschenbroek, the son of a scientific instrument maker, was a medical doctor, mathematician, and natural philosopher who spoke at least seven languages and had attended lectures by Isaac Newton and Newton's experimental assistant, John Theophilus Desaguliers, himself famous for his discoveries regarding the properties of electricity.

Before the discovery of the Leyden jar, electrical experimenters were able to generate electricity using early static generating machines, but they were limited in their experimentation because they had no way to store the electricity thus generated. In 1746, Musschenbroek, working with collaborators, was attempting to electrify water when he got the shock of his life, quite literally:

Musschenbroek attempting to electrify water 

Musschenbroek described his experience in a 1746 letter:
"I wish to inform you of a new, but terrible experiment, which I advise you on no account personally to attempt. I am engaged in a research to determine the strength of electricity. With this object I had suspended by two blue silk threads, a gun barrel, which received electricity by communication from a glass globe which was turned rapidly on its axis by one operator, while another pressed his hands against it. From the opposite end of the gun barrel hung a brass wire, the end of which entered a glass jar, which was partly full of water. This jar I held in my right hand, while with my left I attempted to draw sparks from the gun barrel. Suddenly I received in my right hand a shock of such violence that my whole body was shaken as by a lightning stroke. The vessel, although of glass, was not broken, nor was the hand displaced by commotion: but the arm and body were affected in a manner more terrible than I can express. In a word, I believed that I was done for."
What had happened?

Metal and water conduct electricity, but glass does not. When Musschenbroek's assistant rubbed the rotating glass sphere, a positive static charge was generated. As this positive charge traveled up the chain, across the gun barrel, and down the brass wire into the water, it didn't electrify the water quite as Musschenbroek had hoped. Instead, the static charge continued to travel through the water and built up on the inside surface of the glass jar. 

Simultaneously, a negative static charge was being induced on the outside surface of the glass jar in Musschenbroek's right hand, with his body providing a path to ground. 

These opposite static charges were held in equilibrium on opposite sides of the non-conducting glass until, Zap!, Musschenbroek completed the circuit with his own body by touching with his left hand the inside of the glass jar held in his right hand. The result was violent discharge of stored static electricity, much like a lightning bolt.

Soon after, a London experimenter named Dr. John Bevis replaced the two conductors on either side of the glass (the water inside the jar and Musschenbroek's right hand resting on the outside of the jar) with metal sheets wrapped inside and outside of the jar. A cap was added to the jar to secure a metal rod and chain suspended in the jar. In this configuration, the opposite charges on the inside and outside of the glass jar hold each other in equilibrium until a path is provided for their discharge.

The Leyden jar made it possible for early experimenters to conduct a wide range of electrical experiments.

One experimenter who made excellent use of the Leyden jar was Benjamin Franklin, who was the first to understand and explain how the Leyden jar functions. Franklin based his understanding one of his most important scientific observations—that electrical energy has both positive and negative charges.
Series Pair of Leyden Jars with a total capacity of 450 picofarads at 350 kilovolts

Here at PV Scientific Instruments, we offer a wide range of classic Leyden jar capacitors, from static-electrical experimentation types to spark-oscillation transformer types for radio work. 

Wednesday, March 2, 2011

Lightning Flashing on Saturn

This image from NASA's Cassini spacecraft -- the first of its kind -- shows lightning on Saturn's night side flashing in a cloud that is illuminated by light from Saturn's rings.

The cloud, whose longest dimension is about 3,000 kilometers (1,900 miles), does not change perceptibly over the 16 minutes of observations covered by the 10-second movie. The lightning flashes are the bright spots within the cloud, and are about 300 kilometers in diameter. The lightning strikes last for short periods of time (less than one second before the time line of the movie was compressed).

The energy output of the visible light from the lightning is comparable to the brightest lightning flashes on Earth.

At Saturn, there are three types of clouds that might produce lightning. The top layer is made of ammonia ice; the middle layer is made of a compound of hydrogen sulfide and ammonia; the bottom layer is water. The light has to diffuse up through this cloud system, which is over 100 kilometers (60 miles) thick. The width of the lightning spot at the top of the cloud is proportional to the depth where the flash originated. The observed widths indicate that the lightning is originating either in the hydrogen-sulfide-ammonia cloud or in the water ice cloud. The lightning does not appear to originate at the deepest levels of the cloud system, where water is liquid.

Interested in how early researchers came to understand lightning? PV Scientific offers reprints of classic texts on the subject of atmospheric electricity on our Classic Reprint Page.