By: Brianna Costa, Destiny Iyare, and Nathan Scully Robert Millikan By: Brianna Costa, Destiny Iyare, and Nathan Scully

Previous Model The problem with the previous model of the atom is that no one knew the independent charge of the electron before Millikan. They knew the charge to mass ratio of the electron but only found a rough estimate of the independent charge. J.J. Thomson and others tried to find the independent charge by using clouds and water droplets by seeing how fast they traveled in an electric field but this did not give a good estimate.

Millikan’s Oil-Drop experiment

Millikan’s Oil-Drop experiment cont. Performed in 1909 Used to determine the charge and mass of an electron. Determined the size of the charge on an electron. He put a charge on a tiny drop of oil and measured how strong an applied electric field had to be in order to stop the oil drop from falling. He found out the density of the oil drop, so Millikan and Harvey Fletcher were able to determine the droplets’ masses due to the radii because it was able to calculate the volume.

Millikan’s Oil-Drop experiment cont. The apparatus incorporated two metal plates and a specific type of oil. They used a low vapor pressure because ordinary oil would evaporate under the heat which would cause the mass of the oil to change. An Isolation ring was used to hold the plates apart. The ring had four holes in it, three were for illumination for a bright light to shine through, and another to view through with a microscope. He applied an electric charge between the plates(the bottom charge was negative and the top plate was positive). He could adjust the charge by adjusting the voltage. The oil drops became charged through friction with the nozzle as they were sprayed.

Millikan’s Oil-Drop experiment cont. The oil was dropped into a space between the plates and without the charges turned on the drop would fall freely to the bottom. When the charges turned on he was able to rise the charged particles. More than one drop would come through at a time so he would change the voltage of the charged plates to let all the other drops fell until only one drop remained. His ultimate goal was to have the drop of oil fall at a constant rate.

Evidence/Reasoning Behind Developing A New Model The reasoning behind making a new model was to find out the independent charge of the electron.

New Atomic Model Millikan did not change the overall picture of the model, he just made certain parts more specific. He found that the charge was 1.5924 x 10^-19C which was about 1% smaller from the currently accepted value of 1.6 x 10^-19C. He also found the mass of the electron was 9.04 x 10^-28g

Interesting Facts In 1923 Millikan received the Nobel Prize for his findings. At Oberlin College he taught himself Physics that no one at that college could teach him because they didn't know it. Millikan wanted to rise his academic rank, so that’s why he decided to research the charge of an electron. Millikan proved that radiation is from an ‘extraterrestrial’ (of or from outside the earth) origin, and named it “cosmic rays”. He was chairman of the executive council at Caltech (California Institute of Technology). He retired in 1945.

Websites Used http://www.juliantrubin.com/bigten/millikanoildrop.html http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Ryan_McAllister/Slide3.htm https://www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/atoms-molecules-and-ions-2/discoveries-leading-to-nuclear-atom-model-33/millikan-s-oil-drop-experiment-199-3679/ https://www.aip.org/history/gap/Millikan/Millikan.html http://millikan.weebly.com/fun-facts.html http://www.chemistryexplained.com/Ma-Na/Millikan-Robert.html Oil-drop: http://notes.tyrocity.com/millikan-oil-drop-experiment/