What is Galton's whistle? To what use can it be put?

(N/A) Galton's whistle is a specialized device designed to produce ultrasonic sound waves,typically in the frequency range of $20,000 \,Hz$ to $40,000 \,Hz$.
These high-frequency vibrations are beyond the audible range of human beings,who can generally hear sounds only up to $20,000 \,Hz$.
However,dogs possess a much wider range of hearing and can easily perceive these ultrasonic frequencies.
Because of this,Galton's whistle is used for training dogs to respond to silent commands that humans cannot hear.
It is particularly useful for security purposes; if an intruder enters a property,the whistle can be blown to alert a guard dog without alerting the intruder,allowing the dog to react immediately.