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 OBS.  You should immediately see the feed from the CCD camera.
2. Once you have some beads in video, start recording using the menu on the bottom right.  Be sure to take sufficient data of the freefall of the beads and application of the electric field.  Once you have your data, stop recording.  Your files will be saved in the ‘Videos’ folder and be named with a time stamp.  Immediately change the filename to reflect the experiment that you did.
3. Be sure to take videos of 100, 200 and 300 V applied electric field.

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. Take a very short video so that you can calibrate your pixel measurements to units of meters.