from The Century Dictionary.
- Teeth having one side radial, or nearly so, and the other side much inclined. A pawl, dog, or detent can slip over the inclined side but will catch on the radial side, thus permitting the wheel having such teeth to revolve in one direction only.
Sorry, no etymologies found.
Imagine that at each slight depression between the ratchet-teeth it breaks a contact and cuts off a current, and at each slight rise renews the contact and permits a current to pass.