Question Concerning a Lamp Switch
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. The two-stage lamp switch works as a crude dimmer.

Without more details I'd guess that the switch has a two-stage cam working on three positions of the switch: off - dim - full. The diode forms a crude half-wave rectifier to dim the lamp.

There are two problems with this:


It may cause lamp flicker.
It's drawing DC from the supply transformer which is generally a bad idea as it tends to move the operating point off-centre in the magnetic circuit.