I can’t add anything of value to what has already been said at this ultra-informative write-up from uv201.com:
The model UM-100 “Ultra-Mike” was made by the Mercury Electronic Laboratories of Chicago. It was a wireless microphone which would broadcast to any nearby AM radio. It cost $12.95 when new in the 1950’s. It uses a single 1L4 tube operating as an RF oscillator, modulated by a crystal microphone element. It was powered by a 45 volt B-battery and a single D-cell for the filament. An antenna wire emerged from the bottom of the unit. The frequency could be adjusted to a quiet spot on the AM dial by turning a small red Catalin knob on the side.
The blurb on the carton promised the buyer “an amazing electronic engineering achievement” and a microphone which was “equal in many ways to our present day professional type instruments costing up to $75.00”. It was perfect for the executive, or for use in the nursery, sick room, or factory. It could also be used as a “detectophone” by the enterprising junior detective who wondered what went on in his parents’ bedroom, or between his big sister and her boyfriend. It was “Tomorrow’s Miracle Today”, but wireless mikes very much like it were sold as early as the 1930’s.
The item shown below
is available recently sold on eBay for $99.