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.