Robert Andrew Millikan

Character relationship

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Character experience

After Millikan entered Oberlin College in Ohio in 1886, he was hired as a faculty member in the elementary physics class since the second grade. He liked this job very much, which allowed him to study more deeply. Physics.

After graduating from university in 1891, he continued to teach in the elementary physics class for two years, thus writing a widely circulated textbook. During college, Millikan's favorite subjects were Greek and mathematics.

In 1893, he obtained a master's degree, and in the same year, he received a Ph.D. bonus from the Department of Physics of Columbia University.

In 1895, Millikan graduated with a Ph.D., becoming the first PhD in physics to graduate from the Department of Physics of Columbia University. He then studied at the universities of Berlin and Göttingen in Germany.

In 1896, he returned to China and taught at the University of Chicago. Due to his excellent teaching results, he was promoted to associate professor in the second year.

In 1910, due to his excellent teaching and research work, he was officially promoted to full professor.

In 1921, Millikan left the University of Chicago and transferred to California Institute of Technology as the director of the Normal Bridge Laboratory of the Department of Physics. There, he mainly studied the rays from outer space discovered by another physicist Victor Hess. Millikan proved that these rays did come from outer space and named them "cosmic rays." (Cosmic Rays).

From 1921 to 1945, before Millikan retired, he served as the chairman of the executive board of Caltech. During this period, Caltech became one of the best research universities in the United States.

On December 19, 1953, Millikan died at his home in California at the age of 85 due to a heart attack.

Research results

Determination of elementary charge

Millikan is famous for his experimental accuracy. From the beginning of 1907, he devoted himself to improving the measurement of the charge of alpha particles in the Wilson cloud chamber, which was very effective, which was affirmed by Rutherford. Rutherford advised him to work hard to prevent the water droplets from evaporating.

In 1909, when he prepared the conditions for the charged clouds to increase the voltage to 10,000 volts under the balance of gravity and electric field forces, what he discovered was that "a few water droplets were left in the airport" after the clouds dissipated. , So as to create the balanced water drop method and the balanced slick method to measure the electronic charge, but someone attacked him to get only the average value instead of the elementary charge.

In 1910, he made improvements for the third time, so that the oil droplets can move up and down when the electric field force and gravity are in balance, and the oil caused by the change in electricity can be seen when irradiated. The drop suddenly changes, so as to find the difference of the change in the amount of charge;

In 1913, he obtained the value of the electron charge: e=(4.774±0.009)×10-10esu, in this way, The existence of the elementary charge is confirmed experimentally. The precise value he measured finally ended the debate on the discreteness of electrons and enabled the calculation of many physical constants to obtain higher accuracy.

Planck constant

He is also committed to the study of the photoelectric effect. After careful observation,

In 1916, his experimental results fully affirmed love. Instein's photoelectric effect equation, and measured the most accurate Planck constant h at that time. Because of the above work, Millikan won the 1923 Nobel Prize in Physics.

Elemental Spark Spectroscopy

He also conducted experiments on electrons escaping from the metal surface under the action of a strong electric field. He was also engaged in the research of elemental spark spectroscopy. He measured the spectral region between ultraviolet and X-rays and discovered nearly 1,000 spectral lines, with wavelengths up to 13.66nm), making the ultraviolet spectrum far beyond the known range at the time. His work on the analysis of x-ray spectra led to Uhlenbeek (G. E. Uhlenbeek 1900 ~ 1974) and others to propose the electron spin theory in 1925.

Cosmic rays

He has also done a lot of research on cosmic rays. He came up with the name "cosmic rays". I studied the orbits and curvatures of cosmic particles, and discovered the "alpha particles, high-speed electrons, protons, neutrons, positrons, and V quanta" in the cosmic rays. It changed the concept of "cosmic rays are photons" in the past. Especially for other uses Experimental research on cosmic rays in a cloud chamber in a strong magnetic field led his student Anderson to discover positrons in 1932.

Oil drop experiment

Experimental procedure

The Millikan oil drop experiment, an experiment done by the American physicist Millikan to measure the electric charge.

In 1907-1913, Millikan used charged oil droplets moving in electric and gravitational fields to conduct experiments. It is found that the electricity carried by all oil droplets is an integer multiple of a certain minimum charge. The minimum charge value is the electronic charge. When the oil droplets are sprayed between the two horizontal parallel electrode plates of the capacitor, the oil droplets are sprayed , Is generally charged. In the case of no electric field, a small oil droplet will fall under the action of gravity. When gravity is balanced with the buoyancy and viscous resistance of the air, it will fall at a uniform speed. The relationship between them is: mg=F1+B(1), where: mg──the gravity of the oil droplet, F1──the viscous resistance of air, and B──the buoyancy of air.

Let δ and ρ denote respectively The density of the oil droplets and air; a is the radius of the oil droplets; η is the viscosity coefficient of the air; vg is the uniform drop speed of the oil droplets. Therefore, the gravity of the oil droplets is mg=4/3πa^3δg (Note: a^3 It is the 3rd power of a, all of which are all below), the buoyancy of air mg=4/3πa^3ρg, the viscous resistance of air f1=6πηaVg (Stokes' law of fluid mechanics, Vg represents the subscript g of v). So (1) The formula becomes: 4/3πa^3δg=6πηaVg+4/3πa^3ρg, the radius of the oil droplet a=3(ηVg/2g(δ-ρ))^1/2(2), when When an electric field is applied between the parallel electrode plates, suppose the charge of the oil droplet is q, the electrostatic force it receives is qE, E is the electric field strength between the parallel plates, E=U/d, U is the potential difference between the two plates , D is the distance between the two plates. Properly choose the size and direction of the potential difference U to make the oil droplet move upward under the action of the electric field, and vE represents the rising speed. When the oil droplet rises at a constant speed, the following relationship can be obtained: F2+ mg=qE+B(3), where F2 is the viscous resistance of the air when the rising speed of the oil droplet is Ve: F2=6πηaVe, and the electric quantity q of the oil droplet is obtained from equations (1) and (3) as q=( F1+F2)/E=6πηad/(Vg+Ve)(4). The formula (4) shows that after the radius a of the oil drop is calculated according to the formula (2), the measured drop speed vg when no electric field is applied And when the electric field is applied to the oil droplet's uniform rising speed vE, the amount of electricity q can be obtained. Note that the derivation process of the above formula is for the same oil drop, so for the same oil drop, A set of corresponding data of vg and vE was measured in the experiment. The above method was used to measure many different oil droplets. The results showed that the oil droplets carried The electric quantity is always an integer multiple of a certain minimum fixed value, and this minimum electric charge is the electric quantity e carried by the electron. Connect the instrument to a 220 volt AC power source. Adjust the high-voltage power supply to the 0 position, unscrew the lid of the oil drop chamber, place the level on the upper plate surface, and use the leveling screw to adjust the level of the parallel plate capacitor plate in the oil drop chamber. Adjust the eyepieces of the microscope to make the reticle marks clearly clear. Then insert the pin into the small hole on the upper plate, adjust the angle of the light source, until the light field around the pin is the brightest, the largest range and the light intensity is observed from the microscope, and then pull out the pin and screw on the lid to prepare for spraying. Since the capacitor plates are to be adjusted in this step, beware that the plates are charged and should be adjusted by the teacher. Spray the oil droplets into the oil drop chamber with a sprayer, and observe the movement of the oil droplets from the microscope. In the experiment, first find a suitable oil drop (smaller oil drop, slower movement, less than 5 basic electricity), make it fall freely, and then add an electric field to make it move upward (rising too fast or too much) Adjust the voltage appropriately if it is slow).

In this way, under the alternate action of gravity and electric field force, the oil droplets can rise and fall several times repeatedly, and they can be seen clearly in the entire field of view, otherwise it needs to be reselected. Use stop watch to record: record a certain distance L (distance between the scale of the microscope reticle) of the oil drop n times, the total elapsed time tg, record the total time the oil drop rises the same distance L n times, and the total elapsed time tE total (two records must be for the same oil drop). Divide the total distance nL traveled by the oil drop by the total time tg total and tE total to get vg and vE. Use formula (4) to calculate the oil drop Electricity q. According to the above method, select 6-10 different oil droplets for measurement, and calculate their respective electricity. Data processing: This experiment only requires students to perform simple digital processing and analysis. Record the data and calculations according to the table at the end of the book. This table is a set of data obtained from the experiment with a domestic oil drop meter.

Experimental background

After Thomson discovered the existence of electrons in 1897, people made many attempts to accurately determine its properties. Thomson also measured the specific charge (charge-to-mass ratio) of this elementary particle, confirming that this ratio is unique. Many scientists have done a lot of experimental work to measure the charge of electrons. The precise value of electronic charge was first experimentally measured by American scientist Millikan in 1917. Based on the work of his predecessors, Millikan carried out the measurement of the basic charge e. He made hundreds of measurements. An oil drop had to be stared for several hours, showing the degree of hardship. Millikan used the oil drop experiment to accurately determine the basic charge e.

Significance of the experiment

It is a process of continuously discovering and solving problems. In order to achieve precise measurement, he created the necessary environmental conditions for the experiment, such as the measurement and control of the pressure and temperature of the oil drop chamber. At first, he used water droplets as the carrier of electricity. Due to the evaporation of the water droplets, he could not get satisfactory results. Later, he switched to oil droplets with low volatility.

In the beginning, the e value calculated by the formula from the experimental data increased with the decrease of the oil droplets. In the face of this situation, Millikan analyzed and believed that the reason for this fallacy was that in the experiment The selected oil droplets are very small. For it, air can no longer be regarded as a continuous medium, and Stokes's law is no longer applicable. Therefore, he modified Stokes's law through analysis and experiments and obtained reasonable results. .

Millikan’s experimental device has been continuously improved with the advancement of technology, but its experimental principles are still playing a role at the forefront of contemporary physical science research. For example, scientists have used similar methods to determine Elementary particles-the electricity of quarks. In the oil drop experiment, the ingenious idea and precise idea of ​​converting the micro-quantity measurement into the macro-quantity measurement, as well as the relatively simple instrument, the more accurate and stable results are all enlightening.

Character Evaluation

His realistic, rigorous, and creative experimental style has also become a model in the physics world.