This Westinghouse lag/lead power factor meter is a crossed coil dynamometer type 100 degree scale pivot and jewel bearing meter.
 
Power factor is a measure of how efficiently electrical power is being used in a circuit, expressed as the ratio of "real power" (working power) to "apparent power" (total power drawn), essentially indicating how much of the supplied power is actually doing useful work and not just oscillating back and forth in the circuit; a higher power factor signifies more efficient power usage. 
 
Power factor (PF) is calculated by dividing the real power (measured in watts, W) by the apparent power (measured in volt-amperes, VA).