Perrygraf Load Center Unit-Sub Slide Rule for Allis-Chalmers

This rectangular gray and orange cardboard rule has a cardboard slide and is held together by six metal rivets. The front of the rule explains that the calculator helps users select and test the proper air circuit breakers for electrical substations constructed by Allis-Chalmers. The front of the slide provides reasons why breakers were essential in old and new substation systems. The back of the rule has four logarithmic scales for reading the key number (from the transformer rating), then the index number, then the circuit capacity, and then the amp load. The scales on the slide are color-coded so the user could easily see which level of interrupting capacity was needed.
According to a mark on the slide, Perrygraf Corporation made this instrument, which Allis-Chalmers copyrighted in 1943. While the company is perhaps better known for manufacturing farming equipment, it also built generators, turbines, compressors, and other machinery for heavy industry. Additionally, its engineers designed electrical substations. The company ceased manufacturing operations in 1998. For more on Perrygraf, see 1979.3074.03. For another slide rule made by Perrygraf for Allis-Chalmers, see 1983.3009.04.
Reference: Allis-Chalmers, "Three 'Not-so-Secret' Weapons," The Ohio State Engineer 28, no. 2 (December 1944): 16–17.
Currently not on view
Object Name
calculating rule
slide rule
date made
Perry Graf Corporation
Physical Description
metal (part material)
paper (overall material)
overall: .2 cm x 23 cm x 9.6 cm; 3/32 in x 9 1/16 in x 3 25/32 in
ID Number
catalog number
nonaccession number
Rule, Calculating
Engineering, Electrical
See more items in
Medicine and Science: Mathematics
Science & Mathematics
Slide Rules
Data Source
National Museum of American History, Kenneth E. Behring Center


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