strap-key

from The Century Dictionary.

• noun In experimental psychology, a noiseless break-key used in experiments on tapping. An elastic brass strip, mounted on a wooden block, carries a button for the finger; a platinum point on its upper surface is in contact with a platinum plate on a brass stop fixed above it; and the whole key is incased in a rubber bag packed with felt. A very small pressure on the button breaks the circuit without noise.