Another commercial apparatus containing Mercury atoms was used. The tube contained a cathode, a single grid (\(G_{2}\)), and a collector electrode. The distance between grid 2 and the cathode was relatively large, approximately 8mm, in order to increase the likelihood of collision. The cathode was maintained at a constant voltage, \(6.2V\). Grid 2, which controlled the retarding voltage, was maintained at a constant \(-3.0V\). The Mercury tube was heated to a constant \(185^{\circ}C\). The accelerating voltage was increased in increments of \(2V\). Figure Mercury Fits shows the Franck-Hertz curve for Mercury.