Heinrich Rudolf Hertz (February 22, 1857 – January 1, 1894) was a German physicist who clarified and expanded the electromagnetic theory of light that had been put forth by Maxwell.
He was the first to conclusively prove the existence of electromagnetic waves by engineering instruments to transmit and receive radio pulses using experimental procedures that ruled out all other known wireless phenomena.
Heinrich Hertz’s Wireless Experiment (1887)
In the 1880s many were seeking experimental evidence to establish the equivalence of light and electromagnetic propagation. James Clerk Maxwell’s mathematical theory of 1873 had predicted that electromagnetic disturbances should propagate through space at the speed of light and should exhibit the wave-like characteristics of light propagation.
In 1883 Hertz became a lecturer in theoretical physics at the University of Kiel and two years later he was appointed professor of physics at Karlsruhe Polytechnic. In 1887 Hertz designed a brilliant set of experiments tested Maxwell’s hypothesis. He used an oscillator made of polished brass knobs, each connected to an induction coil and separated by a tiny gap over which sparks could leap. Hertz reasoned that, if Maxwell’s predictions were correct, electromagnetic waves would be transmitted during each series of sparks. To confirm this, Hertz made a simple receiver of looped wire. At the ends of the loop were small knobs separated by a tiny gap. The receiver was placed several yards from the oscillator.
Conceptual Schematic of Hertz’s Experiment
According to theory, if electromagnetic waves were spreading from the oscillator sparks, they would induce a current in the loop that would send sparks across the gap. This occurred when Hertz turned on the oscillator, producing the first transmission and reception of electromagnetic waves.
Hertz also noted that electrical conductors reflect the waves and that they can be focused by concave reflectors. He found that nonconductors allow most of the waves to pass through. Another of his discoveries was the photoelectric effect.
Book by Hertz: Untersuchungen Ueber Die Ausbreitung Der Elektrischen Kraft
(Investigations on the Propagation of Electrical Energy)
[Heinrich Hertz 1892]
Heinrich Hertz was the first to send and receive radio waves. James Clerk Maxwell had mathematically predicted their existence in 1864. Between 1885 and 1889, as a professor of physics at Karlsruhe Polytechnic, he produced electromagnetic waves in the laboratory and measured their wavelength and velocity. He showed that the nature of their reflection and refraction was the same as those of light, confirming that light waves are electromagnetic radiation obeying the Maxwell equations.
Early experimental Hertz radiator and resonator for creating and detecting Hertzian waves
Simple spark gap apparatus similar to this was the first ever built to produce and detect radio waves
All of these findings were first published in the journal Annalen der Physik,(see below right) then in Hertz’s first book, Untersuchungen Ueber Die Ausbreitung Der Elektrischen Kraft (Investigations on the Propagation of Electrical Energy), shown at right. His book is considered to be one of the most important works of science. This is where he first describes his confirmation of the existence of electromagnetic waves.
Annalen der Physik und Chemie is one of the oldest physics journals worldwide. The journal, still in publication today, publishes original papers in the areas of experimental, theoretical, applied and mathematical physics and related areas.
There are 12 complete volumes of Annalen der Physik und Chemie in my collection. Included are Hertz’s many papers proving the Maxwell hypothesis on the propagation of electromagnetic waves. These papers laid the foundation for the development of radio and electromagnetic wave transmission applications. Also included are more Hertz papers plus others by Roentgen, Planck, Boltzmann, Angstrom, Helmholtz.
Today, on the 22 February 2012,The Google logo takes the form of electromagnetic waves (in Google colours – blue, red, yellow and green) to pay tribute to German physicist Heinrich Rudolf Hertz on his 155th birth anniversary. Hertz was born at Hamburg on February 22, 1857.
Hertz was the first to broadcast and receive radio waves. His pioneering work laid the way for the development of radio, television and radar.
The unit of frequency of a radio wave – one cycle per second – is named the hertz, in honour of Heinrich Hertz.
Hertz proved the existence of radio waves in the late 1880s. He used two rods to serve as a receiver and a spark gap as the receiving antennae. Where the waves were picked up, a spark would jump. Hertz showed in his experiments that these signals possessed all of the properties of electromagnetic waves.
With this oscillator, Hertz solved two problems. First, timing English scientis James Clerk Maxwell’s waves. He had demonstrated, in the concrete, what Maxwell had only theorised – that the velocity of radio waves was equal to the velocity of light. (This proved that radio waves were a form of light).
Second, Hertz found out how to make the electric and magnetic fields detach themselves from wires and go free as Maxwell’s waves.
Hertz died at the young age of 36 on New Year’s Day 1894. There is a lunar crater on the dark side of the Moon named after him.
Google has, till the Hertz doodle, posted 1308 doodles on its home page since the first ever Google doodle back on August 30, 1998.
Simplified Circuit for Franck-Hertz Experiment
In an oven-heated vacuum tube containing mercury gas, electrons are emitted by a heated cathode, and then accelerated toward a grid that is at a potential, Va, relative to the cathode. The anode (plate) is at a lower potential, Vp = Va – DV. If electrons have sufficient energy when they reach the grid, some will pass through and reach the anode. They will be measured as current Ic by the ammeter. If the electrons do not have sufficient energy when they reach the grid, they will be slowed by DV, and will fall back to the grid. As long as the electron/molecule collisions are elastic, the collector current depends only on Va and DV since the electrons lose no energy. However, Franck and Hertz discovered that Ic went through a series of maxima and minima as Va was varied. This implies that the gas molecules absorb energy from the electrons only at specific electron energies (resonant energies).
For example, the first excited state of mercury is 4.9 eV above the ground state. This is thus the minimum energy that mercury atoms can absorb from the accelerated electrons. Hence, if VaDV, many electrons pass through the grid and reach the anode, to be measured as Ic. If Va= 4.9 volts, the electrons gain enough kinetic energy to collide inelastically with the mercury atoms just when they reach the grid. In these interactions, the mercury atoms absorb 4.9 eV. Thus, the electrons lose the same amount and no longer have sufficient energy to overcome DV. They fall back to the grid and Ic is a minimum. As Va is raised beyond 4.9 volts, Ic increases again. However, when Va reaches 9.8 volts, the electrons can lose all their energy in two collisions with mercury atoms in two inelastic collisions between the cathode and grid. Again, these are pushed back onto the grid, and Ic falls to a minimum. Current minimum are found whenever Va is a multiple of 4.9 volts.
This simplified description neglects contact potentials. Therefore, Va will need to be somewhat higher than 4.9 volts when the first minimum occurs. Nevertheless, all successive current minima should differ by multiples of 4.9 volts from the first minimum. The spectral frequency corresponding to this energy is 1.18 x 10-15 Hz and the wavelength is 253.7 nm. In their original experiments, Franck and Hertz verified the presence of the ultraviolet radiation with the aid of a quartz spectrometer.
Neon has 10 energy states in the range between 18.3eV and 19.5eV. From these excited levels, the Ne atoms decay to other excited states. These intermediate states decay to the ground state by emitting visible radiation and can be seen in a tube with the room darkened.
Insert a thermometer (0-200 °C) into the top hole of the oven and position the tip to be near the center of the tube. Plug the oven into an appropriate AC power outlet, then turn the thermostat dial to 180 °C. Keep an eye on the thermometer. Do not let the temperature exceed 205 °C.
Connect the tube, control unit, and oscilloscope as follows. Note the German labeling:
M = Plate or Anode A = Grid (not Anode!) H = Filament Heater K = Cathode Ub = Accelerating voltage Va
AFTER the tube has warmed up 10 -15 minutes, switch on the control unit. Set the controls as follows:
Heater = a little less than midrange
Accelerating voltage = fully counterclockwise (Va)
Amplitude = nearly all the way counterclockwise
Reverse bias = fully counterclockwise
Switch Va = ramp (a sawtooth waveform voltage-60 Hz)
After the cathode has warmed up for at least 90 seconds, slowly increase the accelerating voltage amplitude with the control knob. The Franck-Hertz curve should appear on the oscilloscope screen. If necessary, improve the the display by judiciously adjusting the “gain” control and the cathode temperature “filament heating”. Adjust the accelerating voltage such that no self-sustained discharge takes place in the tube. Otherwise, the curve is destroyed by collisional ionization. The accelerating voltage maximum should be about 30V.
The neon tube doesn’t require an external heater but it does have two more connections to be made with the Control unit. The color coding and labeling should be followed. The response is very sensitive to the heater setting – approximately 6.5 seems to work well. Neon has 10 energy states in the range between 18.3eV and 19.5eV. From these excited levels, the Ne atoms decay to other excited states and these states decay to the ground state by emitting visible radiation.
Note that on the oscilloscope trace the vertical deflection is proportional to the anode current Ic, and the input to the x-channel of the oscilloscope is equal to Va.
Use the digital oscilloscope to average traces for both channels. Observe and save the traces in both the dual trace and X-Y modes. Explain why the current peaks are not evenly spaced in the dual trace mode. Now transfer the two time traces to a computer. While you can observe on the oscilloscope the traces in X-Y mode, you will have to transfer the traces separately and then recombine them in other software (preferably Origin) to obtain Ic vs. Va.
Read the data into Origin and plot the anode signal vs. the ramp voltage. On the plot, use the cross-hairs tool to measure values of Va for which the collector current (Ic) is a minimum and compute the separation between adjacent dips. Then tabulate the voltage difference between adjacent maxima. Compute a 90% confidence interval for your results.
You should find that the current minima and maxima are spaced at intervals of ~4.9 volts, showing that the excitation energy of the mercury atom is ~ 4.9 eV. Compare this energy with your 90% confidence interval and explain any deviation.
Repeat the analysis for the Neon tube. You will need a higher Va and reverse bias for this tube.