Arm of the Law is a science fiction short story by Harry Harrison first published in 1958.

How could a robot—a machine, after all—be involved in something like law application and violence? 

Harry Harrison  tells what happens when a police robot hits an outpost on Mars.

A quiet backwater outpost on Mars gets a surprise in the form of a new police recruit - in a box! Yep, it's a prototype robot cop sent to the backwater station for testing.

And Harrison tells the strange, funny and scary things that begin to happen after that, as only he can.

Arm of the Law first appeared in Fantastic Universe August 1958. 

Harry Max Harrison (1925-2012) was an American science fiction author. Harrison is known for his writing, particularly his humorous and satirical science fiction, such as the Stainless Steel Rat series and the novel Bill, the Galactic Hero (which satirises Robert A. Heinlein's Starship Troopers). Harrison is also known for his novel Make Room! Make Room! (1966). The novel was the rough basis for the motion picture Soylent Green (1973).