On this page
Most students meet canal rays as one line in NCERT: discharge in a modified cathode ray tube "led to the discovery of canal rays carrying positively charged particles". Behind that line is a careful 1886 experiment by Eugen Goldstein, and a puzzle about why his magnet failed. This post covers what he saw, why the rays seemed not to bend, a worked example that explains it, and what students can learn from him.
Who was Eugen Goldstein?
Goldstein was born on 5 September 1850 in Gleiwitz, Prussia, and died on 25 December 1930 in Berlin, according to Encyclopaedia Britannica. He spent his career at the Potsdam Observatory, studying electrical discharges in gases. In 1876 he showed that cathode rays cast sharp shadows and leave the cathode at right angles to its surface, which led to curved cathodes that focus the rays.
What did Goldstein see in 1886?
His paper, presented to the Prussian Academy of Sciences by Hermann von Helmholtz on 29 July 1886, carries the title "On a form of radiation at the cathode of induced discharges that has not yet been studied". He used a flat cathode drilled with channels, Kanäle in German, and looked at its back side. Glowing bundles of rays came out through the holes.
He noticed three things:
The colour depended on the gas. The rays were golden yellow in nitrogen, pink in hydrogen, yellowish pink in oxygen and greenish grey-white in carbon dioxide. Because the colour changed from gas to gas, he named them after the channels instead: "Kanalstrahlen", canal rays, until a more suitable name could be found.
The cathode metal did not matter. Platinum, aluminium, copper, steel and brass all gave the same rays.
A magnet did not seem to bend them. Ordinary cathode rays bend easily even near a small hand-held magnet. But even the largest electromagnet in the Berlin Physics Institute, powered by six Bunsen cells, produced "nicht die geringste wahrnehmbare Richtungsänderung", not the slightest visible change of direction.
What were canal rays really?
The Nobel biography of Wilhelm Wien records that in 1898 Wien studied the canal rays discovered by Goldstein and concluded that they were the positive counterpart of the negative cathode rays. NCERT's Class 11 chemistry chapter on the structure of the atom lists their properties: their mass and charge-to-mass ratio depend on the gas, they are bent the opposite way to cathode rays, and the lightest, from hydrogen, is the proton.
Worked example: how much harder are canal rays to bend?
A charge q accelerated through a voltage V and moving across a magnetic field B follows a circle of radius r = mv/(qB). Since ½mv² = qV, this becomes r = √(2mV/q) / B. Use V = 1,000 V and B = 0.010 T; these are sample values, not Goldstein's.
Electron: r = √(2 x 9.109 x 10^-31 x 1,000 / 1.602 x 10^-19) / 0.010 ≈ 1.07 x 10^-2 m, about 1.1 cm.
Hydrogen ion (proton): r = √(2 x 1.673 x 10^-27 x 1,000 / 1.602 x 10^-19) / 0.010 ≈ 0.457 m, about 46 cm.
Ratio: √(1,836) ≈ 42.9. The proton's circle is about 43 times wider.
Nitrogen molecule ion (mass 28 u, single charge): √28 ≈ 5.3 times the proton's radius, about 2.4 m.
A 2.4 m circle bends a beam by only about 2 mm over 10 cm of tube. Goldstein's careful conclusion, that he saw no visible bending, was correct for his setup. The rays were bent, just far too little to see.
Common mistakes to avoid:
Using r = mv/qB with the same speed for both. In a discharge tube, particles get similar energies from the same voltage, not similar speeds. That gives √m, not m.
Forgetting the square root. The radius ratio is √1,836 ≈ 43, not 1,836.
Saying canal rays come from the anode. NCERT notes they are positive ions formed from the gas in the tube, not rays from the anode metal.
For more on circular motion in magnetic fields, see our guide to charged particles in a cyclotron.
How did canal rays lead to isotopes?
The Nobel biography of J. J. Thomson says he found a way to separate different kinds of atoms and molecules using positive rays. His assistant Francis Aston, according to Aston's Nobel biography, first found evidence for two isotopes of neon this way, then invented the mass spectrograph and found 212 naturally occurring isotopes. Our post on Aston's mass spectrograph tells that story.
What can a student take from how Goldstein worked?
Change the apparatus a little. Drilling holes in the cathode revealed rays nobody had looked for.
Name things carefully. He named the rays after the channels, not the colour, because the colour changed with the gas.
"Not seen" is not "not there". His magnet was too weak for heavy ions. Ask whether your test is sensitive enough before ruling something out.
To see the negative-ray side of the same tubes, read our post on William Crookes and cathode rays. If the jump from r = mv/qB to √m feels unclear, the Fermi Problem Library has a step-by-step solver that explains the reasoning.
Next step: work out the radius for a helium ion (mass about 4 u, single charge) in the same field, and check that it is about twice the proton's, near 91 cm.
