4.1 Cautionary Note

Experimentalists are often aware of currently-accepted values of the variables they are measuring. It is very easy to bias oneself in favour of the status-quo. In fact, it’s very likely that measurements of the elementary charge suffered that exact bias. A good experimentalist will do their best to blind themselves to the accuracy of their experiment without losing the ability to make a precise measurement.

4.2 Alignment

1. Start by aligning the light source. Shine the light source through the window in front of it, through the parallel-plate capacitor, and out the window beside the camera. Do not shine the light directly into the camera. SURPRISE, this will blind you to the beads.
2. The camera lens must be adjusted to focus on the short object-distance. The distance selection ring at the front of the lens must be set to less than 0.5m, and the aperture, which is the back ring, to f/11. Or whatever ends up working best.  You’ll figure it out.  Don’t be afraid to change the lens settings BEFORE you start collecting data.
3. Disconnect the atomizer and stick the metal lead of a resistor down the hole in the top plate. By adjusting the position of the camera along its track, you should be able to get a well-focused image of the resistor lead. Make small adjustments to the lens if you need to.

4.3 Adding ‘Oil Droplets’

1. Remove the resistor and plug the atomizer back in. Open the white container and make sure that the small nozzle of the aerator is submerged in the solution of polystyrene spheres. If not, find a TA to add some for you.
2. With the beads ready, and the atomizer connected to the parallel plate capacitor, squeeze the atomizer bulb. If the camera is properly focused, you should see a flurry of beads enter the parallel plate capacitor. If you don’t immediately see the beads, there is likely an issue with the camera focus or light alignment. Make small adjustments to the position of the camera and/or angle of the light source. You can also adjust the ‘black level’ setting on the monitor, but that’s less likely. This alignment is difficult and may take several tries. If you STILL can’t see any beads, then ask your TA for assistance.  

4.4 Measuring ‘Oil Droplet’ Velocity

1. Open the Python program to access the video feed. Use the right and left mouse buttons to move the cursors on the screen. The program will display the number of pixels between the cursors, thereby giving you a measure of the distance the beads travel. Combined with a measurement of the elapsed time, one can determine the velocity of the bead.
2. Before applying an electric field, you need to determine the free-fall velocity of the beads. Try to measure the freefall of a single bead 5-7 times.  The stdev of those measurements will be your uncertainty in going forward. 
3. Apply a potential across the parallel plate capacitor. You can choose 100 V, 200 V, or 300 V – whichever you think would be best. You’ll see the beads move vertically, some up and some down (how is it that they can move in both directions?). Try to spot some of the slower-moving beads. Ideally, you will take 5-7 measurements of the same bead you used for the free fall measurement so that you have an estimate of the uncertainty in the same way.
4. Once you have a few measurements of a single bead, you only need one measurement per bead (well, two, you need and ).
5. According to Equation (4), is linearly proportional to , the voltage applied to the parallel plate capacitor. Experimentally determine the validity of that proportionality by measuring and  with the other two values of applied voltage to the parallel plate capacitor. Take as many measurements as you have time for. More measurements of more spheres will make for a more precise result.

4.5 Calibrating your Measurements

1. So far, all of your velocity measurements are in units of pixels/s. That unit is mostly useless.
2. Without adjusting the lens, slide the camera backwards along the track while holding a ruler near the window until the markings on the ruler are in focus.
3. Use the cursors to determine how many pixels are in a mm of the measuring tape. You will have some uncertainty in this value. Use this calibration factor to translate your velocity measurements into units of m/s.