Robert Millikan's apparatus which first established the quantitization and value of the electric charge.

Data acquisition was by telescopic observation of drops, timed by a stop watch as a manually operated knife switch was used to change the electric field.

Drop generation was by generation of a mist via an atomizer. A single drop was selected from this mist by the human observer.

Millikan's total mass throughput was about a hundred drops.

Designed and built at San Francisco State University, this was the first automated Millikan apparatus based fractional charge search which tested large amounts of bulk matter.

Data acquisition was by a photomultiplier tube generating pulses from laser illuminated drops which were converted to velocities by digital hardware counters.

Drop generation was by a drop on demand piezoelectrically driven drawn capillary tube ejector.

The mass throughput was on the order of milligrams (millions of drops).

Fractional charge search experiment utilizing a CCD video camera and personal computer. Measurement accuracy achieved was 1/40 e.

Drop generation utilized a laser machined metal aperture, drop on demand drop ejector.

Mass through was about 1 milligram of silicon oil (6 million drops).

The primary purpose of this experiment was to establish that low cost commercial quality computer video hardware could be used to take accurate charge and mass measurements.

The fractional charge search at SLAC  1996-1999

The drop ejector is a glass and silicon micromachined drop on demand ejector capable of producing programmable arrays of 7-10 micron diameter target drops.

This experiment's technical goals are to:  Measure an array of drops falling simultaneously between the electric field plates. The ability to use a video camera to image and measure the individual charges of a falling array of drops is the key to achieving extremely high mass throughput.

The final mass through put was 17.4 milligrams of silicon oil (40 million drops)

Measurement accuracy achieved was 1/50 of an electron charge.

The fractional charge search experiment at SLAC (1999 - 2001)

The drop ejector is a glass and silicon micromachined drop on demand ejector producing 20 micron diameter target drops.

This experiment's technical goals were to:

• Measure the charges of multiple drops falling simultaneously between laterally oriented electric field plates.
• Increase mass throughput over the conventional Millikan geometry by utilizing large drops slowed by an upwards laminar air flow to allow multiple charge measurements .

The final mass through put was 70.1 milligrams of silicon oil (17 million drops)

Charge measurement accuracy achieved is 1/50 of an electron charge.

The fractional charge search experiment at SLAC (2004 - 2007 )

The drop ejector is a glass and silicon micromachined drop on demand ejector producing 25 micron diameter target drops from a suspension of carbonaceous chondrite meteoric material.

The use of a mineral suspension as a test material has made the required hardware more sophisticated than in the prior experiments:

• Due to slow settling out of the suspension, the drop ejector had to be monitored by a machine vision system in order to appropriately change the electronic drive to compensate.
• The meteorite suspension, because of the high ion density compared to pure silicone oil, ejected drops with thousands of electron charges which had to be brought near neutrality by passage through ionized air in order to be accurately measured.
• Slow drifts in the diameters of the generated microdrops over time required implementation of machine vision mediated auto-controls to stabilize the average charge of the drops, the average fall paths of the drops and the magnitude of the upwards laminar air flow.

This experiment's scientific goals were to:

• Search for fractional electric charge in primordial material that has not undergone geologic, biological or industrial processing.
• Develop the technology needed to test for fractional electrically charged matter placed into fluid suspension using the Millikan method of charge measurement.

The final mass throughput was 262.8 milligrams of meteorite in oil suspension (42 million drops).
The meteorite mass fraction in the suspension was 1.5 percent by weight.

Charge measurement accuracy achieved is 1/24 of an electron charge.