To calculate the amplitude ratios, the amplitude of the fundamental frequency and several higher harmonics are selected, with the number of usable harmonics based on the fidelity of the square wave and the sampling rate of the detector. Once the harmonics are extracted, the amplitude ratio is calculated by where AR is amplitude ratio, A is amplitude, and n is frequency, such that for a given square wave 1 is the fundamental frequency, and 3, 5, etc., are the third harmonic, fifth harmonic, and so on. In this work, harmonics up to the seventh were used, meaning that up to four data points were collected for each single experiment. The greater the number of harmonics, the more scattered the amplitude data becomes, and though, in principle, many more than four harmonics can be used, four were chosen to minimize scatter in the data, as described below.