The gauge may be used to measure the strength of a cell in a battery used for electric clock systems during the period the first electric clocks were introduced in the United States. Without dismantling the pocket watch size meter with the snap-back removed, two small series connected electromagnets can be seen. The  pole pieces are arranged to form a flux gap.  A curved armature pivoted on the the axis of a toothed segment is attracted to the flux gap when the electromagnets are energized. A spiral hair-spring opposes the rotation of the toothed segment carrying the armature .  The toothed segment engages a  pinion mounted on the axis of the pointer. Experiment with a single flashlight cell causes the needle pointer to rotate clockwise about forty-five degrees pointing to the middle of the scale. I believe the gauge likely has a full scale reading of less than three volts based on the logic of the electrical potential of the  LeClanche (zinc-carbon) cells or the lead-acid cells commonly making up telephone and clock system batteries of the pre-900 industrial period.    Note the serial number, 2634 on the face and compare with number 22 in the advertisement.   This would suggest that Warner’s Electric Gauge was a widely used service  tool in primary battery systems. The gauge was listed on eBay. I bid and lost. Several days later, while visiting Dave O’Dell, I noticed the gauge on his workbench.  The identity of winning auction bidder J3442’s  was revealed.  Thanks Dave.