Butz Thermostat

Build the whole loop at 12 V or less and use a lamp or small heater as the "fire". Never wire a home-made control to a real furnace, boiler or flue — that is regulated work for good reasons.

Butz claims two circuits: one that works the damper as temperature rises and falls, and one that sounds an alarm when the house gets cold enough to mean the fire needs fuel.

Watch a thermometer and switch a lamp on and off by hand to hold a target temperature. Time how long you can keep it steady. You are the controller, and you are the part being replaced.

Bond a strip of steel to a strip of brass along their length. Unequal expansion makes the pair bend when heated — that bend is the measurement.

Warm the strip and record deflection against temperature at four points. This curve is what turns a piece of metal into an instrument.

Mount a contact screw so the strip touches it at a chosen temperature. Moving the screw moves the set point — this is the thermostat dial.

Put a meter across the contact and warm the strip. It goes from open to closed. It reports nothing about how cold the room is — only whether it is past the line.

Wire the contact to a coil wound on a soft-iron core. The tiny force of a bending strip now commands a much larger force — the strip does not have to move the damper itself.

Wind a spring that wants to swing a flap, held by a latch. Let the electromagnet trip the latch. Butz uses stored mechanical energy released electrically, not an electric motor.

Mount a card or metal flap that restricts airflow to your lamp enclosure. Open flap, more heat; closed flap, less.

Connect everything and leave it alone for ten minutes. Temperature rises, contact opens, damper shuts, temperature falls, damper reopens. Nobody is watching it.

Record temperature every 30 seconds. It does not sit at the set point — it swings above and below it. This is normal for on/off control and it never fully goes away.

Put the strip close to the lamp and re-run. It now switches off almost immediately and the room stays cold. A controller measures where its sensor is, which is why thermostat placement is not decoration.

Fit a second contact further along, so a much lower temperature rings a buzzer. That is the alarm: the fire is losing, come and feed it.

The patent. US 341,092, "Thermo-electric damper-regulator and alarm", filed 14 December 1885 and granted 4 May 1886 to Albert M. Butz of Minneapolis. It was known as the damper flapper, and the company Butz founded to sell it became, after a long chain of mergers and renamings, Honeywell.

📌 This blueprint completes a three-part arc across this collection, and the distinction is worth holding onto. Warren Johnson's electric tele-thermoscope of 1883 INDICATES: it reports the temperature elsewhere and does nothing about it. Seely's electric flat-iron of 1882 REGULATES: a series resistance sets how much heat is produced, but nothing measures the result, so the temperature it reaches depends on the room. Butz CONTROLS: the measurement acts back on the source, and the loop is closed. Sensing, regulating and controlling are three different problems, and the nineteenth century solved them in that order — the first two are useless on their own for holding a house at a temperature, and the third needs neither better sensors nor better heaters, only a connection between them.

Why a spring motor rather than an electric one. In 1885 there was no domestic electricity worth the name; Butz's circuit runs on a battery, which can supply a brief pulse to an electromagnet but not sustained work. So the energy to move the damper is stored mechanically in a wound spring, and electricity is used only to decide — to trip a latch. This is the standard shape of early control engineering: a weak, cheap signal releasing a strong, stored force. The same division exists in a modern system, where a thermostat switches milliamps and a relay or valve does the work.

Butz sold out early and missed it all. In 1888 he assigned the patent to his own patent attorneys, who formed a company to exploit it; he was out within a few years of inventing it. The business passed through the Consolidated Temperature Controlling Company and the Minneapolis Heat Regulator Company before the 1927 merger that produced Minneapolis-Honeywell, and the corporate name became Honeywell in 1964. It is the same shape as Ingersoll and Langstroth elsewhere in this programme — the invention outlived the inventor's interest in it by about eighty years.

The oscillation in step 12 never got fixed, only managed. Simple on/off control must overshoot, because the heat already in the system keeps arriving after the contact opens. Adding deliberate hysteresis stops the contact chattering; anticipators, and later proportional and PID control, reduce the swing. But a domestic thermostat today still cycles around its set point rather than sitting on it, and the plot you draw in step 12 is recognisably the plot of a modern heating system.