1. This is an electrical rule with scales for volts and for the efficiency of dynamos and motors in the well on the stock.
2. The rule is very similar to the PIC 121 and has all the scales in the same layout as that rule. The additions are the scales in the well of the stock, the naming of the A and B scales as "10 amps" and "10 yd. - .01 Sq. In" and the modification of the slide with a Perspex tongue added.
3. This rule is a successor to the PIC 4866 electrical rule but has a number of differences including the Perspex tongue rather than  metal tongue for use with the scales in the well and the use of the patented differential trig scale arrangement rather than "traditional " trig scales on the back of the slide of the 4866.
4. Both this rule and the 4866 do have the patented differential log-log scales for values close to 1.00.