Perrygraf Ohm's Law Calculator Slide Rule for Ohmite

<< >>
This rectangular yellow, orange, black, and white rule consists of a paper envelope holding a paper slide, held together with metal rivets. The Ohmite Manufacturing Company of Chicago commissioned it from the Perrygraf (or Perry Graf) Corporation of Maywood, Ill.
Lester E. Perry (1901–1991), a machinery inspector in Cook County, Ill., founded Perrygraf in 1934. He and his associates, including his relative Oscar Perry, designed and manufactured special-purpose "slide charts." By 1960, the firm had a branch in California and a plant in Puerto Rico. In 1968 the firm was sold to Nashua Corporation, and operations in Illinois ceased. Some former employees started their own slide chart companies. James E. Johnson, the former plant manager, established American Slide Chart in 1971. This Wheaton, Ill., firm became sufficiently successful to purchase Perrygraf in 1988. Another former Perrygraf employee and engineer established Datalizer Slide Charts of Addison, Ill. Both American Slide Chart/Perrygraf and Datalizer Slide Charts remain in business as of 2012.
The Ohmite Manufacturing Company, established in 1925, sold rheostats, fixed resistors, and adjustable resistors. Its Ohm's Law Calculator had scales that allowed the user to find ohms, volts, amperes, and watts, given any two of those measures. It also had a chart for finding the stock number of desired parts. This example was copyrighted in 1941 and sold for ten cents. The name of the donor is stamped on both sides. An electronic version of the calculator may be found on the company's website.
References: George Melloan, "Pocket Slide Charts Aid Engineers, Help Sell Steel, Lipstick," Wall Street Journal, September 4, 1953, 1; Display Ad, Los Angeles Times, November 8, 1960, 22; "About Perrygraf,"; "About Us," American Slide Chart/Perrygraf,; "Slide Chart Specialists," Datalizer Slide Charts,; "Ohmite History,"
Currently not on view
date made
Perry Graf Corporation
Physical Description
paper (overall material)
overall: .3 cm x 23.5 cm x 10.6 cm; 1/8 in x 9 1/4 in x 4 3/16 in
ID Number
nonaccession number
catalog number
Credit Line
Gift of Oscar W. Richards
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


Add a comment about this object