4.1 Regarding the Program Options

• The NIST team was nice enough to write a program in LabView that can operate the Watt Balance. It shows graphs of relevant variables and provides user-friendly calibration and measurement programs. You really need the currents and coil heights graphs. You can control the range of the y-axis by just typing in new min and max values.
• The first thing you should do once the program is started and graphs are open is go the the ‘Settings’ tab and change the polarity of the coils. They both need to be changed to ‘negative’ to account for the direction that the wire is wound around its frame.
• We’ll be in ‘auto’ mode the entire time, so don’t worry about the various ratings for manual difficulty.
• The most important sub-programs for your measurement will be the zero-position and SS calibration. The rest of the measurement will be trivial if these two parts are done well.
• The final tab on the right has options that were in progress at the time that the program was shared; they are not functional.

4.2 Regarding the Measurements

• I humbly suggest you use Coil A (the one on the left) as the driving coil and put the weight on Coil B (the right-side pan). Then drive the Coil B to measure  of Coil A.
• Unfortunately, friction exists. And sometimes the program that controls the balance has a hard time getting to a balance position with an accuracy much better than a human hair. If you’re balance is oscillating around the set position with an amplitude of about 0.02 mm, consider how you might estimate the current using the min and max values of that incredibly small oscillation.
• The crux of this apparatus is the shadow sensor. Using geometry, it determines the vertical position of the weighing pan and its derivative determines the velocity. The NIST paper shows a very linear shadow sensor calibration centered on the balanced, level position of the Watt Balance. Proper, accurate calibration of the shadow sensor is INCREDIBLY important. Use a range of only 1.5 mm. Too large a range results in some nonlinear behaviour near the edges of the shadow sensor. 5 points should give you a decently-precise calibration.
• The program that determines the weighed mass and Planck’s constant asks the user to measure the zero position, then the position with the mass, then the zero position again. The program then calculates the mass for you. It’s a gimmick for lay-people. We need more precision than that. Measure the mass according to the technique shown in Figure 8 in the NIST LEGO Watt Balance reference.
• The measurement programs hide pretty much all of the math from you. It’s important to understand what you are measuring at each step and how your results are being determined. Your report is judged on how well you present your data and interpret your results, not on the value of a handful of numbers that a computer generated for you.
• You’ll have a 2-gram mass to test out the Watt balance with. Your results should be accurate to about 10% and precise to about 1%. Then you can measure an unknown mass using Planck’s constant definitions. Measure the mass of a penny or two and see if you can determine approximately when they were manufactured.

4.3 Tips for your Report

• Lab reports in this class focus on data presentation, analysis, and interpretation. You should have an introductory section in your report to give your results some context, but please do not recreate the NIST paper in your own words.
• Include as many plots as you can. LabView can usually save data directly from the UI display, or the program has an option to save it to the hard drive.
• As the NIST paper states, it’s relatively straight forward to arrive at a measured mass or at a value for Planck’s constant. The hard part is properly characterizing the uncertainty in those values.