Millikan Oil Drop Apparatus
Millikan Oil Drop Apparatus
The oil drop experiment was performed by Robert A. Millikan and Harvey Fletcher in 1909 to measure the elementary electric charge (the charge of the electron). The experiment took place in the Ryerson Physical Laboratory at the University of Chicago. Millikan received the Nobel Prize in Physics in 1923.
The experiment entailed observing tiny electrically charged droplets of oil located between two parallel metal surfaces, forming the plates of a capacitor. The plates were oriented horizontally, with one plate above the other. A mist of atomized oil drops was introduced through a small hole in the top plate and was ionized by x-rays, making them negatively charged. First, with zero applied electric field, the velocity of a falling droplet was measured. At terminal velocity, the drag force equals the gravitational force. As both forces depend on the radius in different ways, the radius of the droplet, and therefore the mass and gravitational force, could be determined (using the known density of the oil). Next, a voltage inducing an electric field was applied between the plates and adjusted until the drops were suspended in mechanical equilibrium, indicating that the electrical force and the gravitational force were in balance. Using the known electric field, Millikan and Fletcher could determine the charge on the oil droplet. By repeating the experiment for many droplets, they confirmed that the charges were all small integer multiples of a certain base value, which was found to be 1.5924(17)×10−19 C, about 0.6% difference from the currently accepted value of 1.602176634×10−19 C. They proposed that this was the magnitude of the negative charge of a single electron.
This apparatus is an experimental device used in Secondary and Higher Education. From High School to University and Polytechnic schools and institutions of higher education. It can test and verify the quantum nature of charges, measure elementary charges and observe Brownian motion.
The device consists of a the oil drop box, measuring microscope and power supply. The power supply unit is located inside the casing while the oil drop box and the microscope are installed on the casing.
Inside the oil box there are two parallel round plates insulated each other with a distance of d, placed in a transparent plastic hurricane globe. There is a small hole in the middle of the upper polar plate. After spraying oil drops from the spraying hole into the oil box , some mist from oil drops falls into the small hole on the upper polar plate and enter the chamber between the upper and lower polar plates. The illumination is carried out by means of a 4.8V focusing beam peanut bulb.
The measuring microscope is placed in front of the oil drop box. The lighted oil drop can be observed from the microscope. There is division scale in the eye lens. Its divisions are equivalent to 2mm of the field of vision used to measure the distance L of oil drop uniform motion so as to find the velocity of oil drop motion.
Power supply: 4.8V power is obtained directly from the transformer, used for oil drop illuminating.
0-500V DC power supply is obtained through bridge rectification and filtering by capacitor to provide the polar plates with continuously adjustable voltage.
Specifications:
Input voltage: 220VAC, 50Hz
Output power: 5W
Operating voltage between upper and lower polar plates: 0-500V
Distance between polar plates: 5 ±0.2mm
Total magnification of measuring microscope: 30x
overall dimensions: 320mm x 220mm x 190mm