Millikan’s experiment

C02-0107. In discovering the electron, Thomson was able to measure the ratio of the electron s mass to its charge, but he was unable to determine the mass of a single electron. How did Millikan s experiment allow the mass of the electron to be determined ... 

Millikan s experiments wich single oil drops, beginning in 1906, provided a method for measuring extremely small charges with precision. He was able to show that the charge on his drops was always ne. with e = 1,60 x 10 19 coulomb (modem value) and n a positive or negative integer. 

Millikan s experiment did not prove, of course, that (he charge on the cathode ray. beta ray, photoelectric, or Zeeman particle was e. But if we call all such particles electrons, and assume that they have e/m = 1.76 x Hi" coulombs/kg. and e = 1.60 x 10" coulomb (and hence m =9.1 x 10 " kg), we find that they fit very well into Bohr s theory of the hydrogen atom and successive, more comprehensive atomic theories, into Richardson s equations for thermionic emission, into Fermi s theory of beta decay, and so on. In other words, a whole web of modem theory and experiment defines the electron. The best current value of e = (1.60206 0.00003) x 10 g coulomb. 

To understand this discrepancy, we need to remember that there is a second source of error in any experiment systematic error that causes a shift in the measured values from the true value and reduces the accuracy of the result. By making more measurements, we can reduce the uncertainty due to random errors and improve the precision of our result however, if systematic errors are present, the average value will continue to deviate from the true value. Such systematic errors may result from a miscalibration of the experimental apparatus or from a fundamental inadequacy in the technique for measuring a property. In the case of Millikan s experiment, the then-accepted value for the viscosity of air (used in calculating the charge e) was subsequently found to be wrong. This caused his results to be systematically too high. 

In Millikan s experiment these fi-ee electrons became attached to some of the oil droplets. 

In practice, Avogadro s number can be determined by electrochemical experiments like this. The charge of the electron can be found independently by Millikan s experiment. 

Millikan s experiment is discussed in most first year physics textbooks, the original experiment is described in R. A. Millikan, A New Modification of the Qoud Method of Determining the Elementary Electric Charge and the Most Probable Value of that Charge, Philos. Magazine, 19 (1910) 209 - 228. 

C02-0041. In Millikan s oil drop experiment, some droplets have negative charges, so others must have... 

If any oil droplets in Millikan s oil drop experiment had possessed a deficiency of electrons, the droplets would have been positively charged and would have been attracted to, not repelled by, the negatively charged plate. There would have been no voltage setting possible where the electrical and gravitational forces on the drop would have balanced. 

This is only about 1/1836 the mass of a hydrogen atom, the lightest of all atoms. Millikan s simple oil-drop experiment stands as one of the cleverest, yet most fundamental, of all classic scientific experiments. It was the first experiment to suggest that atoms contain integral numbers of electrons we now know this to be true. 

Figure 2.5 Millikan s oil-drop experiment for measuring an electron s charge. The motion of a given oil droplet depends on the variation in electric field and the total charge on the droplet, which depends in turn on the number of attached electrons. Mi Ilikan reasoned that the total charge must be some whole-number multiple of the charge of the electron.

A number of investigators over the years have determined the values for the numerical coefficients used in the expression above. Allen and Raabe (1982) have reanalyzed Millikan s data (based on experiments performed between 1909 and 1923) to produce the updated set of parameters shown above. 

Millikan s oil-drop experiment settled the argument and determined accurately (within one part in a thonsand) both the charge and, by virtue of the charge-to-mass ratio, the mass of the electron. Both munbers allowed the Danish physicist Niels Bohr to finally calcnlate Rydberg s constant and provided the first and most important proof of the new atomic theory. 

A) Crooke s cathode-ray tube with a Maltese cross target to cast a shadow demonstrating the particle behavior of electrons B) Thomson s cathode-ray deflection experiment for measuring m/e of the electron C) Millikan s oil-drop experiment that determined the charge (e) and mass (m) of the electron... 

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