1. This rule was designed for electrical calculations. Two versions of the same rules, one from 1937 and one from 1961 are presented. It was made in Germany specifically for the British market.

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. On the 1961 version, 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. On the 1937 version there was a table of comparative resistance.

6. Although basically identical, there are some minor differences between the two rules:

• The cursor on the 1937 rules is made of aluminium and brass: the 1961 one of plastic.
• The scales are given the "standard" lettering (A, B, LL1 etc) on the later rule but are almost unnamed on the earlier rule.
• Only the earlier rule has a table of resistance.
• The earlier rule is "Made in Bavaria": the later one "Made in Germany".
• The earlier rule is marked1/98/398: the later one 1/98 Elektro.
• Pins are used to help fix the celluloid scales to the stock on the earlier rule: on the later rule they are only used to fix the metal "tongue".

7.  Instructions for use can be found here.