The article describes and figures the construction and use of a portable electromagnetic percussion hammer operated by a direct current motor; also a device for recording the knee jerk. The advantages of the apparatus are: it is automatic, easy to operate, gives a long continuous record; the hammer gives a constant blow at constant intervals, both of which can be varied, and the hammer always strikes the same place. The apparatus can be used on man, dogs, and cats for studying various factors influencing reflex muscular response and conduction through the motor arcs.