What is an ancillary device?
An ancillary device is one that is not the primary one. For
example in your motor car, the propulsion system is the primary
device, otherwise there would be no purpose in having it.
But the lamps are ancillary devices, but pretty important
nonetheless. But your radio or cigar lighter are much less
important, and are ancillary devices.
Similarly with your cooking stove, the clock is likely an
ancillary device and you use it rarely, if at all. This clock is an
ancillary device.