1. This rule was designed for electrical calculations.
2. An unusual feature (used virtually exclusively for electrical rules) is the presence of scales in the well of the stock whose values were read by a small metal tongue. These scales were for efficiency of motors and dynamos and voltage loss.
3. On the front of the rule there was temperature scale to allow for the variation of electrical resistance with temperature.
4. As can be seen on the diagram of the detail on the well of the stock, this rule was set up for units of 10 amp, 10 yds and 10000 circular mils. (A "circular mil" is the square of the diameter of a wire in thousands on an inch.).
5. Few of the tables on the back were specifically electrical except for the one showing the use of gauge marks for length, weight, resistance and diameter..
6.  Instructions for use can be found here.