Modern Physics

Anode rays: discovery, properties

The anode beams or channel beams , also called positive positive beams are rays consisting of atomic or molecular cations (positively charged ions) which are directed towards the negative electrode in a Crookes tube.

Anode rays originate when electrons going from the cathode to the anode collide with the atoms of the gas contained in the Crookes tube.

As the particles of the same sign repel each other, the electrons that go to the anode tear the electrons present in the crust of the gas atoms.

Thus, atoms that remained positively charged – that is, were transformed into positive ions (cations) – are attracted to the cathode (with a negative charge).


It was the German physicist Eugen Goldstein who discovered them, observing them for the first time in 1886.

Later, work on anode rays by scientists Wilhelm Wien and Joseph John Thomson eventually took over the development of mass spectrometry.


The main properties of anode rays are as follows:

– They have a positive charge, the charge value being an integer multiple of the electron charge (1.6 ± 10-19 C).

– They move in a straight line in the absence of electric and magnetic fields.

– They deviate in the presence of electric and magnetic fields, moving towards the negative zone.

– Thin layers of metal can penetrate.

– They can ionize gases.

– Both the mass and the charge of the particles that make up the anode rays vary according to the gas contained in the tube. Usually their mass is identical to the mass of the atoms or molecules from which they are derived.

– They can cause physical and chemical changes.

A little of history

Before the discovery of anode rays, there was the discovery of cathode rays, which occurred throughout the years 1858 and 1859. The discovery is due to Julius Plücker, mathematician and physicist of German origin.

Later, it was the English physicist Joseph John Thomson who studied in depth the behavior, characteristics and effects of cathode rays.

For his part, Eugen Goldstein – who had already done other research with cathode rays – was the one who discovered anode rays. The discovery took place in 1886 and he made it when he realized that the discharge tubes with the perforated cathode also emit light at the end of the cathode.

In this way he discovered that, in addition to cathode rays, there were other rays: anode rays; these were moving in the opposite direction. As these rays passed through the cathode holes or channels, he decided to call them the channel rays.

However, it was not he but Wilhelm Wien who later carried out extensive studies of anode rays. Wien, along with Joseph John Thomson, ended up laying the foundations for mass spectrometry.

Eugen Goldstein’s discovery of anode rays was a fundamental pillar for the subsequent development of contemporary physics.

Thanks to the discovery of anode rays, fast and orderly swarms of atoms were available for the first time, whose application proved to be very fertile for different branches of atomic physics.

the anode ray tube

In discovering anode rays, Goldstein used a discharge tube that pierced the cathode. The detailed process by which anode rays are formed in a gas discharge tube is set out below.

By applying a large potential difference of several thousand volts to the tube, the electric field created accelerates the small number of ions always present in a gas and created by natural processes such as radioactivity.

These accelerated ions collide with the atoms in the gas, ripping off electrons and creating more positive ions. In turn, these ions and electrons attack more atoms again, creating more positive ions in what is a chain reaction.

Positive ions are attracted to the negative cathode and some pass through holes in the cathode. When they reach the cathode, they have already accelerated at a sufficient speed that, when they collide with other atoms and molecules of the gas, they excite the species to higher levels of energy.

When these species return to their original energy levels, atoms and molecules release the energy they had previously gained; Energy is emitted in the form of light.

This light-producing process, called fluorescence, causes a glow to appear in the region where ions emerge from the cathode.


Although Goldstein, with his experiments with anode rays, obtained protons, the truth is that he is not the one who attributes the discovery of the proton, as he was not able to identify it correctly.

The proton is the lightest particle of the positive particles that are produced in anode ray tubes. The proton is produced when the tube is charged with hydrogen gas. In this way, when hydrogen ionizes and loses its electron, protons are obtained.

The proton has a mass of 1.67 × 10 -24 g, about the same as the hydrogen atom, and has the same charge but opposite sign as the electron; that is, 1.6 × 10-19 C.

Mass spectrometry

Mass spectrometry, developed from the discovery of anode rays, is an analytical procedure that makes it possible to study the chemical composition of the molecules of a substance based on their mass.

It allows you to recognize unknown compounds, count known compounds, and know the properties and structure of a substance’s molecules.

In turn, the mass spectrometer is a device with which the structure of different chemical compounds and isotopes can be analyzed very accurately.

The mass spectrometer allows atomic nuclei to be separated based on the relationship between mass and charge.

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