A vacuum-type thermionic diode, designed for student laboratory use, is described which features variable electrode spacing, interchangeable electrodes, and a guard-ring structure. Experiments discussed include a measurement of emitter and collector work junctions, a study of the dependence of space-charge effects on electrode spacing, and field emission studies. Results are generally reproducible and in reasonable agreement with other published data. A space-charge correlation originally developed by Houston and Webster is tested against experimental data for the first time, and a unique measurement of the Schottky parameter at elevated temperature is reported.