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History
Electricity in Thin Air
Geissler and Crookes tubes make electrical activity visible inside glass
By the middle of the nineteenth century, experimenters had batteries, induction coils, and generators capable of producing useful electrical potentials. Glassblowers and instrument makers were also becoming much better at removing air from sealed glass vessels.
Put those developments together and a new kind of electrical device appears: the gas-discharge tube.
it could make the passage of electricity through a thin gas visible.
Geissler tubes — 1850s
The German glassblower and instrument maker Heinrich Geissler developed improved vacuum pumps and produced sealed glass tubes containing gases at reduced pressure. When a sufficiently high voltage was applied between electrodes in the tube, the gas glowed.
Different gases, pressures, tube shapes, and voltages produced different colors and patterns. These were laboratory instruments, but they also showed that electrical conduction did not have to take place only through solid metal wires or liquid electrolytes.
The tube is not really empty
The word vacuum can be misleading. Early discharge tubes still contained some gas. As the pressure was reduced, the electrical behavior changed dramatically. At some pressures the tube glowed strongly; at still lower pressures the glow changed character and new effects appeared near the electrodes.
Cathode and anode
The negative electrode became known as the cathode, while the positive electrode was called the anode. These names will become extremely important later in vacuum tubes, diodes, batteries, electrolysis, and semiconductor devices.
Crookes tubes — 1870s
English scientist William Crookes experimented with tubes evacuated to much lower pressures. He observed effects that seemed to originate at the cathode and travel through the tube. They could produce fluorescence where they struck the glass and could cast shadows when an object was placed in their path.
These mysterious emissions became known as cathode rays. At the time, physicists argued over what they actually were. Were they some kind of wave, particles, or an effect of the remaining gas? That question would remain open for years.
- a sealed glass envelope;
- electrodes brought through the glass;
- low gas pressure or a strong vacuum;
- a high electrical potential between electrodes;
- and a visible response inside the tube.
A beam that can be studied
Cathode-ray experiments showed that whatever was leaving the cathode traveled in a directional way through the tube. Magnetic fields could influence its path, and fluorescent materials could make its arrival visible.
That combination — a source, a beam, a way to steer it, and a glowing target — is the ancestor of devices that will later become the cathode-ray tube, oscilloscope, radar display, and television picture tube.
Measurement trail 14 — voltage is not the whole story
A metal resistor can often be ball-parked with Ohm's law once its resistance is known. A gas-discharge tube is different. Its behavior depends strongly on pressure and on whether the gas has already been ionized.
Why the first spark matters
Before ionization, the gas may behave almost like an insulator. Once a discharge begins, free charges inside the gas can make further conduction much easier. That is why many gas-discharge devices have a distinct starting condition and a different running condition.
The vacuum tube is not yet an electronic amplifier
Crookes tubes were extraordinarily important, but they did not yet provide the controlled thermionic current that later vacuum-tube electronics would use. Their cathodes were not the heated electron emitters of Fleming's diode or de Forest's triode.