Application of photo-electric cells to timing with a standard clock

The purpose of this problem is to work out a simple arrangement whereby signals may be obtained from a standard clock, at any time, without disturbing the clock in any way. This arrangement proposes that a mirror be placed on the pendulum, and after the pendulum has been adjusted to take care of this added mass, the mirror can be left in place permanently. Whether it is being used or not, the mirror causes no noticeable change in the period of the clock.