# Which oil was used by Robert A. Millikan in his oil drop experiment?

I need to know which oil was used by Robert A. Millikan in his oil drop experiment, i.e.name of oil or chemical formula of the oil molecule.

• CH3(CH2)4CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOH – Count Iblis Nov 11 '16 at 22:55
• What prevented you from performing some basic research on the subject? – Lightness Races in Orbit Nov 12 '16 at 3:34
• @LightnessRacesinOrbit - In fairness to the OP, I spent thirty seconds googling the question and didn't immediately find the answer in the first two links. – Richard Nov 12 '16 at 9:26
• @Count Ibis Thanks, from where you get this chemical formula (reference ?) ? – VYT Nov 12 '16 at 10:08
• That seems to be the formula for en.wikipedia.org/wiki/Arachidonic_acid. I doubt very much that the oil was composed of a single fatty acid, especially one usually found in animals. – Level River St Nov 12 '16 at 10:50

The name, as far as I can find, is "highest grade of clock-oil" as mentioned in his 1913 paper (page 111, last line). The density of this oil at $23\, {}^\circ \mathrm C$ is $0.9199$, but the units aren't mentioned.

EDIT: I went to look up an earlier work (1911) and found two other oils of interest that were used, namely "cleaned gas engine oil" of density $0.9041$ and the more volatile mineral oil (machine oil) of density $0.8960$, both measured at $25\, {}^\circ \mathrm C$. I believe these are the oils the Wikipedia entry alluded to.

• Which means, sadly for the OP, that an exact name or formula is not known without becoming a detective in to the state of the clock-oil market at the time. Likely just a nice mineral oil (which itself is not necessarily all one molecule). – Jon Custer Nov 11 '16 at 15:19
• @NaOH Thanks! Jon Custer is right. At least, the clock-oil used today can give a hint. – VYT Nov 11 '16 at 15:33
• Density is almost certainly going to be specific gravity compared to water. At what tempurature and pressure, who knows... – Yakk Nov 11 '16 at 18:29

A fine mist of oil droplets was sprayed into a chamber above the plates. The oil was of a type usually used in vacuum apparatus and was chosen because it had an extremely low vapour pressure. Ordinary oil would evaporate under the heat of the light source causing the mass of the oil drop to change over the course of the experiment. Some oil drops became electrically charged through friction with the nozzle as they were sprayed. Alternatively, charging could be brought about by including an ionising radiation source (such as anX-ray tube). The droplets entered the space between the plates and, because they were charged, could be made to rise and fall by changing the voltage across the plates.

Paraffin oil has a vapor pressure of 0.5 kPa, as a benchmark. I think the original paper should say. I am not an experimentalist, but I will add experimental physics to your tag list.

I think, not surprisingly, it might be a variety of sperm oil. If you really need to know more , Ken Reindel runs Ken's Clock clinc kensclockclinic.com and it looks like he knows, if anybody does, as he is a specialist for that era.

• Thanks, vacuum pump oil used in the past is very unclear answer, I need to know chemical formula. If this is a mix, at least what is the average number of carbon atoms for the oil molecule. – VYT Nov 11 '16 at 15:16
• Given the time, I think sperm oil is an excellent guess (and unfortunately for the OP, now essentially unobtainable). – Jerry Coffin Nov 11 '16 at 17:20
• High grade neatsfoot oil (see britannica.com/topic/neats-foot-oil ) was also used for clocks, and is still available now. – abligh Nov 11 '16 at 17:26