Cathode rays Lenard's major contribution to physics was in the study of
cathode rays, which he began in 1888. Prior to his work, cathode rays were produced in primitive, partially evacuated glass tubes that had metallic electrodes in them, across which a
high voltage could be placed. Cathode rays were difficult to study using this arrangement, because they were inside sealed glass tubes, difficult to access, and because the rays were in the presence of air molecules. He overcame these problems by devising a method of making small metallic windows in the glass that were thick enough to be able to withstand the pressure differences, but thin enough to allow passage of the rays. Having made a window for the rays, he could pass them out into the laboratory, or, alternatively, into another chamber that was completely evacuated. These windows have come to be known as
Lenard windows. He was able to conveniently detect the rays and measure their intensity by means of paper sheets coated with phosphorescent and materials. In particular, he came to use pentadecylparatolylketone, which was very effective as a cathode ray detector but, unfortunately for Lenard, not fluorescent in
X-rays. When
Wilhelm Röntgen set out to reproduce Lenard's results, he was forced to use
barium platinocyanide instead because Lenard had purchased all the available pentadecyl-para-tolyl ketone. The alternative was sensitive to both
UV and X-rays allowing Röntgen to discover X-rays. Lenard observed that the absorption of cathode rays was, to first order, proportional to the density of the material they were made to pass through. This appeared to contradict the idea that they were some sort of
electromagnetic radiation. He also showed that the rays could pass through some inches of air of a normal density, and appeared to be scattered by it, implying that they must be particles that were even smaller than the molecules in air. He confirmed some of
J. J. Thomson's work, which eventually arrived at the understanding that cathode rays were streams of negatively charged energetic particles. He called them
quanta of electricity or for short
quanta, after
Helmholtz, while Thomson proposed the name "corpuscles", but eventually
electrons became the everyday term. In conjunction with his and other earlier experiments on the absorption of the rays in metals, the general realization that electrons were constituent parts of the atom enabled him to claim correctly that for the most part atoms consist of empty space. He proposed that every
atom consists of empty space and electrically
neutral corpuscules called "dynamids," each consisting of an electron and an equal positive charge. As a result of his
Crookes tube investigations, he showed that the rays produced by irradiating metals in a vacuum with ultraviolet light were similar in many respects to cathode rays. His most important observation was that the energy of the rays in the
photoelectric effect was independent of the light intensity. His interpretation however imagined that the light released rays already moving inside of atoms and he made no connection between the energy of the light and the electron. These latter observations were
explained by
Albert Einstein as a quantum effect. Each quantum of light with sufficient energy resulted in one photoelectron, so the light intensity affected the electron flux intensity but not its energy. This theory predicted that the plot of the cathode ray energy versus the frequency would be a straight line with a slope equal to the
Planck constant,
h. This was shown to be the case some years later. The photoelectric quantum theory was the work cited when Einstein was awarded the Nobel Prize in Physics in 1921. Suspicious of the general adulation of Einstein, he became a prominent
skeptic of relativity and of Einstein's theories generally; he did not, however, dispute Einstein's explanation of the photoelectric effect. Lenard grew extremely resentful of the credit accorded to Wilhelm Röntgen, who received the first
Nobel Prize in Physics in 1901 for the discovery of X-rays. Lenard wrote that he, not Röntgen, was the "mother of the X-rays," since he had invented the apparatus used to produce them; Lenard likened Röntgen's role to that of a "midwife" who merely assists with the birth.
Meteorological contributions Lenard was the first person to study what has been termed the
Lenard effect in 1892. This is the separation of electric charges accompanying the
aerodynamic breakup of water drops. It is also known as
spray electrification or the
waterfall effect. Lenard conducted studies on the size and shape distributions of raindrops and constructed a novel
wind tunnel in which water droplets of various sizes could be held stationary for a few seconds. He was the first to recognize that large raindrops are not tear-shaped, but are rather shaped something like a hamburger bun. ==
Deutsche Physik ==