These links describe a variety of devices available for improving old electric clocks, or for making your own.
An electronic Master Clock for old slave dials --- turns slave dials into functioning clocks
or will synchronize Western Union and Self Winding clocks at the hour
An electronic Master Clock for antique European slave dials like French Brillie and ATO
This version outputs the alternating polarity required by many Eureopean slaves
An electronic battery that makes Eureka, Bulle, and Brillie clocks keep quartz accurate time
A way to make antique battery clocks more reliable
A complete electronics package for making your own accurate electromagnetic clock