When the new Birmingham physics laboratory was being set up by Poynting in 1908 these were the most precise clocks available, with a claimed precision of 0.05 seconds per day. The special feature of these master clocks is that the pendulum is suspended by a leaf spring below a table that rocks on a pair of knife edges (image d). The escape mechanism operates from this table leaving the pendulum undisturbed. The pendulum is made of invar and a device is mounted on it to compensate for changes in barometric pressure. The invar pendulum does not change dimensions with temperature changes so the clock is able to keep more precise time. Power is delivered through a falling arm (visible through a hole in the face) and this arm is reset by a pulse from a battery every 20 to 25 seconds. The clock was originally synchronised to a slave clock and used to deliver timing pulses around the laboratory.

In 2014 the clock was not running well and was then restored in 2016 by the Clockworks of London.

Inscriptions / Translations:  On dial: S. Riefler München
DRPNo 50739 DRPNo 100870
No 245 1908
On pendulum bob: RIEFLER
MÜNCHEN
DRP 100870
779
On barometric unit: No. 35