[Image above: Charles R. Pratt in his office, early 1900s (AC0958-0000009). Courtesy Archives Center, National Museum of American History, Smithsonian Institution]
On May 21, 1921, the elevator in the Commerce Trust Building in Kansas City, Missouri stopped on the seventeenth floor and opened. A man stepped into the car. He rode the elevator, equipped with a Pratt equalizer, up and down and noted, “The rope equalizers are working perfectly and have not given a bit of trouble. I never before really appreciated the real value of an equalizer until I rode on top of the car up and down the hoist way. Grasping the ropes in my hands I was able actually to feel the equalizer functioning in its effort to distribute equally the strain of the six ropes.” The man on the elevator was John Heslip, a colleague and co-inventor with Charles R. Pratt (1860-1935), of a cable tension equalizing device patented on November 30, 1923 (US 1,474,911) and assigned solely to Pratt.
Pratt, a mechanical engineer who graduated from the Massachusetts Institute of Technology (1879), combined the mechanical operation of the hydraulic elevator engine with the more efficient operation of the electric motor. He began his career with the Whittier Machine Company of Boston (1880-1883) as a chief draughtsman, and in 1889 he designed, built ,and installed—in conjunction with the Whittier Machine Company—an electric elevator. In 1892, Pratt joined Frank Sprague, a fellow inventor with interests in electric motors, at the Sprague Electric Elevator Company. There Pratt and Sprague developed the Sprague-Pratt Electric Elevator. In 1898 Sprague Electric became part of the Otis Brothers Elevator Company that would dominate the elevator industry.
Pratt’s work on elevators was well-known. He was retained by the Commerce Trust Building and its managers, Henrici-Lowry Engineers, from October 9, 1920 to February 24, 1921 to design the rebuild of seven passenger elevators, originally designed and sold by Edward E. Wright (b. 1861), a mechanic and millwright from Detroit with elevator experience. Wright had suggested that Pratt be brought on to the project. Along with Wright and other draughtsman from Henrici-Lowry Engineers, they sought to correct the issues surrounding rebuilding the elevator. Pratt’s original drawings were used by Henrici-Lowry under Wright’s daily observation.
Pratt’s “equalizing” patent (US 1,474,911), filed on April 6, 1921 was intended “to provide an improved apparatus whereby the tension on several load carrying cables may be maintained substantially uniform while operating.” The invention would be most valuable to elevators whose load needed to be properly distributed. Pratt wrote to Heslip on March 29, 1921, “It is hard to estimate the commercial value of this patent but I will make the best quick cash deal that I can, and let you in right in appreciation of your efficient and courteous assistance in my work on the Commerce Trust Building elevators.” Pratt would later estimate in June 1925 that the equalizer could “easily earn a profit of $100,000 a year.”
To complicate matters for Pratt, Wright was negotiating for patent sales with a working model of Pratt’s equalizer that he claimed to own. Wright patented an equalizer (US 1,465,705) on August 21, 1923—three months before Pratt. Wright suggested to Pratt that he abandon his pursuit of filing a patent as he was the rightful inventor. Wright began an patent interference with the United States Patent and Trademark Office, Edward E. Wright v. Charles R. Pratt (Interference No. 48, 278) on November 16, 1922. An interference proceeding, also known as a priority contest, is a proceeding before patent judges to determine the priority issues of multiple patent applications.
According to Pratt’s records—specifically, his sketches and fiber board template—he and Heslip conceived of the idea in Kansas City first, implementing their invention in the Commerce Trust Building. Pratt’s November 30, 1920, sketch, made on the back of a menu for the Hotel Baltimore in Kansas City, details his elevator rope equalizer. According to Pratt’s archives, there is no evidence that Wright supplied his drawings and sketches.
Pratt and Heslip wasted little time in collecting the necessary evidence to prove their clear title as the real and only inventors. On June 2, 1921, Pratt wrote by registered mail to Mr. W. Malcolm Lowry in Kansas City, “As the battle is now on, please let me know when you start the barrage at Mr. Howard Cox, and its effect." Howard Cox was the attorney representing Wright and, in yet another twist, a cousin to Mr. Lowry of Henrici-Lowry Engineers.
By January 1923, Pratt and Wright seemed ready and willing to resolve the issue of the patent interference. In a January 23, 1923 letter to Howard M. Cox, Pratt wrote, “I cannot conscientiously bring myself to concede priority to Wright.” Pratt suggested “a practical businesslike” agreement that would have Wright pay Pratt $500 in cash; Pratt would no longer pursue the patent interference; and Wright would grant Pratt an exclusive license to manufacture, use, and sell all forms of the equalizer except the form shown in Wright's patent. By March 16, 1923, Pratt and Wright had sealed the deal and the agreement, license, and check were duly executed.
Pratt’s archive of personal and work papers detailing his elevator ups and downs and other inventive endeavors is available at the Archives Center.
- Middleton, William D. and William D. Middleton IIII. Frank Julian Sprague: Electrical Inventor and Engineer. Indiana University Press. 2009.
- Lindquist, David. “Modern Electric Elevator and Elevator Problems, Mechanical Engineering, Volume 37, June 1915, pages 309-333.