J. J. Thompson

Amber and Fur

Our ancestors noticed that when amber was rubbed against fur it attracted small objects...why is this? The knowledge of static electricity dates back to the earliest civilizations, but for millennia it remained merely an interesting and mystifying phenomenon, without a theory to explain its behavior and often confused with magnetism

Barometric light

Evangelista Torricelli invented the barometer in 1643. Barometric light was first observed in 1675 by the French astronomer Jean Picard "Towards the year 1676, Monsieur Picard was transporting his barometer from the Observatory to Port Saint Michel during the night, [when] he noticed a light in a part of the tube where the mercury was moving; this phenomenon having surprised him, he immediately reported it to a Journal

In order to produce barometric light, the glass tube must be very clean and the mercury must be pure. The Swiss mathematician Johann Bernoulli studied the phenomenon while teaching at Groningen, the Netherlands, and in 1700 he demonstrated the phenomenon to the French Academy. After learning of the phenomenon from Bernoulli, the Englishman Francis Hauksbee investigated the subject extensively.[8] Hauksbee showed that a complete vacuum was not essential to the phenomenon, for the same glow was apparent when mercury was shaken with air only partially rarefied, and that even without using the barometric tube, bulbs containing low-pressure gases could be made to glow via externally applied static electricity

Ironically, his father (who was a book seller) intended him to be an engineer, which in those days required an apprenticeship, but his family could not raise the necessary fee. Instead young Thomson attended College, which had an excellent science faculty and was then recommended to Trinity College where he studied Mathematics on scholarship, Cambridge. He was then appointed as a Professor of Physics! This seems pretty unusual as he had personally done very little experimental work and his focus had been in math not physics.

But J.J. brought a fresh perspective into physics. He was using his mathematical expertise to look into the chemical composition of gases. He could develop the consequences of a theory by mathematical analysis.

Gas dicharge demonstrations

Even though he was clumsy with his hands, he had a genius for designing apparatus and diagnosing its problems.

In 1802, Sir Humphry Davy demonstrated in the same year the electric arc at the Royal Institution of Great Britain . Power is drawn from the banks of batteries in the basement and rapidly used up by the intense light. Electric light was then only a scientific curiosity,

Electricity was probably exotic at that time, and was perceived as an amusement.

Our air is not normally electrically conducting, nor are gases in general. But as a gas is exhausted from a tube, until only a residue is left, it loses this character. Then a discharge of electricity will make it glow in splendid colors! Each type of gas with its own delicate shade.

As the pressure drops, dark gaps begin to appear in mysterious ways. At that time the pressure could be lowered to one thousandth of the atmospheric pressure. This was done using an air pump. Boyle, Hooke, Huygens, Guericke had all spent much time developing this instrument. There was really no striking improvement in the design of the device for 150 years. Only when this antiquated instrument could be replaced by a revolutionary device could the progress resume.

Heinrich Geissler was a skilled glassblower from a glass art making family in Thuringen Germany and worked as a travelling instrument maker in Germany and the Netherlands. Geissler opened a small company in Bonn in 1852 to sell his self made scientific glass instruments to schools and Universities

In that same time he developed a new type of pump - the mercury vacuum pump. With this instrument he was able to create an higher vacuum (2 Torr) than possible that time with standard equipment - the piston air pump. Geissler was able to pump and seal glass tubes so it was much more easy to use. This was a big improvement over instruments like "electric eggs" because of the better pump and better seal.

German glassblower Heinrich Geissler, who beginning in 1857 constructed colorful artistic cold cathode tubes with different gases in them which glowed with many different colors, called Geissler tubes. It was found that inert gases like the noble gases neon, argon, krypton or xenon, as well as carbon dioxide worked well in tubes. This technology was commercialized by French engineer Georges Claude in 1910 and became neon lighting, used in neon signs.

On a June afternoon in 1752, the sky began to darken over the city of Philadelphia. Franklin had been waiting for an opportunity like this. He wanted to demonstrate the electrical nature of lightning, and to do so, he needed a thunderstorm

To dispel another myth, Franklin’s kite was not struck by lightning. If it had been, he probably would have been electrocuted, experts say. Instead, the kite picked up the ambient electrical charge from the storm

Despite a common misconception, Benjamin Franklin did not discover electricity during this experiment—or at all, for that matter. Electrical forces had been recognized for more than a thousand years, and scientists had worked extensively with static electricity. Franklin’s experiment demonstrated the connection between lightning and electricity.

But even without understanding how the Leyden jar worked, several improvements were made to it. Rather than using the glass itself as both the bearer of charge and the insulator between the two types of charge, William Watson lined the glass jar inside and out with metal foil; the glass then served principally as an insulator. The copper wire through which charge was entering was attached directly to the foil by a metal wire rather than by water.

Friction machines

By the end of the 17th century, researchers had developed practical means of generating electricity by friction, but the development of electrostatic machines did not begin in earnest until the 18th century, when they became fundamental instruments in the studies about the new science of electricity.

How do materials behave in vacuum?

Now what are the implications of these dramatic decreases in pressure? Well for one thing, a material having a certain volatility on the surface of the earth continually ejects molecules which are driven or rather bounced back to the surface of material from whence it originated. Indeed, unless acted upon by an outside agency, e.g. heat, light, chemical energy, etc., the surface of the material will be in eqyilibrium such that the number of molecules which leave the surface will equal those that return. Needless to say, as the pressure surrounding the material is decreased, the probability of collisions between the gaseous molecules, constituting the ambient pressure,and the ejected “materials” molecules becomes increasingly improbable. Hence, those molecules that leave the surface will travel farther before colliding and returning. In a very high vacuum, there are so few gaseous molecules that the ejected molecules will not return at all.


Hence, we now have the circumstance that, for all practical purposes, molecules leaving a surface donot return and the material sublimes, i.e. goes from the solid to the vapor phase directly (by omitting or by-passing the liquid stage) at a rate dependent upon its vapor pressure

In 1885, Sir William Crookes carried out several experiments to study behaviour of heated metal in vacuum. He found that cathode produces a stream of radiation, which could cause a glow in gases at low pressure. He called these radiations coming out of cathode as cathode rays. Then he studied the behaviour in the discharge tube.

Fun with high voltages