content_image/364663.jpg
<img src='content_image/364663.jpg'>

By using two measured frequencies, ω$_{l}$ and ω$_{n}$ , rather than a single frequency, we obtain a measurement of c ′ 0 independent of the surface tension. Fig. 3 (left panel) shows the values of c ′ 0 that we obtain from the above ‘two-frequency’ method using the lowest order mode n = 2 and a second mode l > 2. We use the n = 2 mode since the corresponding peak in the power spectrum is always clearly resolved. We plot the mean of the values obtained for each mode l > 2. Error bars (standard error) indicate the variation in the measured c ′ 0 obtained from different modes l . Filled circles and open squares show data obtained from a drop in a nitrogen atmosphere at B$_{0}$ = 16 . 5 T, and in air at B$_{0}$ = 16 . 2 T, respectively. The broken line on Fig. 3 shows the value of c ′ $_{0}$( r ) computed from the potential U shown in Fig. 1 (i.e. from the geometry of the solenoid and the current B$_{0}$ ). This line is in reasonably good agreement with the experimentally measured values of c ′ $_{0}$, although the data points fall at slightly higher values. As an additional check of our method, we plot the dependence of the measured c ′ 0 on l (right hand panel). If the trap-induced frequency enhancement that we measure experimentally is accurately given by Eqn. 11, then the c ′ 0 values that we determine by this method should exhibit no dependence on l . Although there is some scatter in the 

