Electric Tele-Thermoscope

Johnson claims a device that signals whether conditions are too warm or too cold. It indicates. It does not open a damper or command a boiler. Indicating and regulating are different jobs.

Rivet or braze a brass strip to a steel strip along their length. Brass expands roughly twice as much as steel per degree, and that mismatch is the entire sensor.

The two strips must be bonded continuously. Fastened only at the ends they slide past each other and the strip stays flat instead of bending.

Warm it with a hot-air gun. It bows towards the steel side, because the brass has grown longer and the pair must curve to accommodate the difference.

Wind it into a spiral. Deflection scales with the SQUARE of length, so coiling a long strip into a small space turns a barely visible bow into a large rotation.

Fix one end rigidly and attach a pointer to the other. All the accumulated bending now appears as angular travel at the free end.

Mark the scale at known temperatures using a reference thermometer. An uncalibrated bimetallic coil indicates change, not temperature.

Place an adjustable contact above and below the rest position. Touching one means too warm, the other too cold — and the gap between them is the dead band.

Move the contacts closer and the device becomes twitchy, chattering on tiny fluctuations. Wider is calmer but less responsive — this trade-off never goes away.

Run wires to two lamps or an annunciator on a board. This is the tele- part: the reading is now available somewhere the room is not.

Add a second sensor on its own pair of wires. One person now sees the whole building — which is exactly the labour Johnson was trying to eliminate.

Watch with a meter as the pointer arrives. A bimetallic strip moves slowly, so contacts touch lightly and can arc or bounce.

Fit a small magnet near the contact so the pointer is pulled through the last of its travel. It now snaps closed and open — the fix for slow, dirty switching.

Wire a contact to a relay driving a damper motor. The moment the signal acts on the heat source, you have crossed from indication to regulation — and that step is not in this patent.

The patent, precisely. US 281,884, "Electric tele-thermoscope", granted 24 July 1883 to Warren S. Johnson of Whitewater, Wisconsin. It describes a coiled spring of two or more metals of different expansibility that moves with temperature and closes electrical contacts to signal too warm or too cold at a remote point. It is a remote INDICATOR. Johnson is very widely described as the inventor of the electric room thermostat, and he did found the Johnson Electric Service Company — later Johnson Controls — but this particular number is not a thermostat, and it is worth getting right rather than repeating the shorthand.

Why two metals bend. Every metal has a coefficient of thermal expansion; brass is roughly twice steel's. Bond them face to face and heat them, and the brass tries to become longer than the steel it is stuck to. The only shape that accommodates both is a curve, with the brass on the outside. Deflection is proportional to the temperature change, to the difference in expansion coefficients, and to the SQUARE of the strip's length — which is why the practical form is a long strip coiled into a spiral, converting a tiny differential into a visible rotation.

The dead band is a design parameter, not a defect. Two contacts either side of neutral define a range in which nothing is reported. Narrow it and the device chatters on every draught; widen it and the building drifts further before anyone knows. Every on-off controller since has the same parameter — it is the hysteresis in a modern thermostat, and the reason your heating does not cycle every thirty seconds.

Where the distinction leads. A sensor plus an indicator is an open loop: it informs a human, who decides. Connect the same contacts to an actuator on the heat source and you have closed the loop, and the system regulates itself. That is the step from Johnson's tele-thermoscope to the pneumatic room thermostats his company later built and installed across American schools and offices. Bimetallic strips remain everywhere — in thermal circuit breakers, kettle cut-outs, oven thermostats and old car flashers — and every one of them is doing what this coil does: turning a difference in expansion into a mechanical decision.