The availability of low cost digital multimeters capable of measuring capacitance has made parallel plate capacitor investigations common in the introductory laboratory. Typically, students add multiple thin sheets of dielectric material between conducting plates. The capacitance is measured and then plotted versus the reciprocal of the dielectric thickness ͑the nominal plate separation͒. We explain why the experiment fails for small dielectric thickness and discuss an improved version of the experiment using dielectric sheets of various thicknesses rather than multiple thin sheets of a dielectric.

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To help students develop an understanding of the proper use and function of spectrographs and monochromators we describe a student-assembled spectrograph using a ‘webcam’ detector. The apparatus also works well as a low-cost demonstration, helping students make connections between an atomic spectrum observed by eye and a plot of the relative intensity versus wavelength. It also allows students to observe the internal components of a spectrograph.

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We report the observation of bound–free emission on the 1 3Π→1 3Σ+ band of the NaK molecule. The spectra, which consist of oscillating continua in the near-infrared, have been analyzed to determine parameters describing the repulsive wall of the 1  3Σ+ state above the dissociation limit. Spectra calculated using a potential of the form Ae−BR +C for the 1 3Σ+ state were compared to experimental spectra to yield the following values: A=5.94×105 cm−1, B=1.605 Å−1, C=−220.520 cm−1. This potential, which is referenced to the bottom of the RKR 1 3Σ+ well (De =209.1 cm−1), is valid over the range R=3.4–4.5 Å (R=6.4–8.5 a.u.). The relative transition dipole moment of the 1 3Π→1 3Σ+ band has also been determined over a limited range in R (7.5<R<8.9 a.u.) through the study of relative intensities of various maxima within each oscillating spectrum. In the simulated spectra, the dipole moment was represented by a functional form D(R)=m(R−R0)+D0 where D0 was used to normalize the results to a recent theoretical calculation (D0=1.07 a.u. R0=8.034 a.u.). The best fit for the parameter m was determined to be 0.121±0.029 in atomic units.