The present clock was invented and made by Captain E.E. Craig. It and its sister clock (the only other in existance) were taken to the Greenwich trials in 1939 and both were found to be highly accurate. The sister clock was subsequently given to the Science museum Inv. 1949/193 where it is on display to this day. The working description reads as follows;
The pendulum receives its impulse from a pair of light steel balls which roll down two short ramps attached to the pendulum by horizontal arms. The pendulum has no unlocking duty to perform and swings freely except while it is actually receiving its impulse and for the small fraction of the second during which the ball rests against the edge of the ramp before rolling down it.
After the right hand ball has imparted its impulse to the pendulum it rolls away toward the back of the clock movement and operates a trigger which allows the right hand mercury switch to tilt over and close its circuit. The right hand electro magnet operates and the resulting movement throws the right hand ball upward to its highest position and at the same time releases the small catch holding the left hand ball. This ball then rolls down a small fraction of an inch until it rests against the edge of the left hand pendulum ramp which is then on its downward travel. As the ramp descends the ball is able to roll down it impart its impulse and then roll away towards the back of the clock movement. The second half of the cycle is similar, but with right and left interchanged.