"Be it known that I, Warren S. Johnson, of Whitewater, in the county of Walworth, and in the State of Wisconsin, have invented certain new and useful Improvements in Electric Tele-Thermoscopes; and I do hereby declare that the following is a full, clear, and exact description thereof.

picture from patent filing

Patent drawing of Johnson's
bimetal electric thermostat

"My invention relates to devices adapted to indicate, at any convenient point, the relative temperature in rooms, conservatories, cellars, & etc., situated at a point remote from the indicators of my device; and it consists of certain peculiarities of construction, as will be more fully set forth hereinafter."

So begins the 1883 patent description (full patent, pdf) of the bimetal-actuated electric thermostat which was the original invention of Warren Johnson, a professor at State Normal College in Whitewater. Receiving the patent led leading to the founding of the Johnson Electric Manufacturing Company in Milwaukee with the backing of hotelier and packing heir William Plankinton. That invention, show at the right, was the first of several from the future Johnson Controls produced that today control the systems that keep us comfortable.

A rapid succession of improvements followed. In 1895 Warren Johnson patented a pneumatic thermostat that was the first application for regulating heating systems and subsequently became an industry standard.

The earliest thermostats, in use before the pneumatic invention of Johnson, were annunciators used to alert a boiler operator to conditions elsewhere in a building. When methods of direct fluid control-were developed, they often employed a combination of electric and pneumatic/hydraulic devices. An electric switch closure, for example, might control a pneumatic valve through a solenoid relay or an expanding fluid might directly actuate a valve through a capillary tube. The complexity or bulkiness of such methods limited the range of application of temperature control.

The pneumatic thermostat patented by Mr. Johnson uses a bimetallic element to control airflow through a nozzle and thereby operate a pilot regulator. The amplified air signal is conveyed through a single tube to a steam or hot water valve on a heat exchanger.

In February of 1906, the novelty of this approach was successfully defended in Federal Court.

Johnson subsequently invented the ‘Humidostat’, a word he coined. Patented in 1909, it was the first humidity sensing element used in a building air control system. The expansion of a maple dowel as it absorbed moisture from the surrounding air was used to control the leakage of air from a pneumatic port. The amplified signal was then used to control an apparatus for the addition of moisture to the air.

In recognition of Mr. Johnson's achievements, in 2008 ASME recognized an installation in Milwaukee as a Historic Mechanical Engineering Landmark.

Next - Charles Taylor supplies air to mining operations without using mechanical pumps.