How Do You Measure the Temperature of a Furnace? Two Wires and a Voltage

A wire with one end hotter than the other develops a small voltage along its length, because heat drives charge carriers from the hot end towards the cold one until an opposing electrical force balances the flow. The size of that voltage differs between metals. Joining two different metals at one end and measuring across the two free ends therefore gives a net voltage that depends on the temperature difference between the joined end and the free ends, since the two contributions do not cancel. That voltage is small, measured in millivolts, and its relationship to temperature is well characterised for standard metal pairs, so measuring it gives the temperature directly.

Several properties make it the default for industrial measurement:

• •A range from far below freezing to well over a thousand degrees
• •A junction that can be made extremely small, so it responds in milliseconds
• •No power supply, since the device generates its own signal
• •Physical toughness, being two welded wires rather than a delicate element
• •Very low cost compared with other sensors of comparable range
• •Long leads possible, so the instrument sits far from the heat

Because the voltage depends on the difference between the two ends rather than on the hot end alone, knowing the hot temperature requires knowing the cold one, which is the central practical complication. Early practice held the free ends in melting ice to fix them at zero, which is accurate and impractical outside a laboratory. Modern instruments measure the temperature at their own terminals with a separate simple sensor and correct the reading arithmetically, which is called cold junction compensation and is built into every commercial device. Getting it wrong is a common source of error, and it is why extension wires must be made of the same metals as the sensor rather than ordinary copper, since a joint in the wrong metal creates an unintended second junction.

Several sensors compete and knowing their trade-offs explains which appears where. A resistance thermometer uses a platinum element whose resistance changes predictably with temperature, and it is considerably more accurate and more stable than a thermocouple over a moderate range, which is why laboratory and calibration work uses one. A thermistor uses a semiconductor whose resistance changes sharply with temperature, giving high sensitivity over a narrow range at very low cost, which suits domestic appliances. An infrared thermometer measures radiation emitted by a surface without touching it, which is the only option for a moving or inaccessible object and which depends on knowing how well that surface radiates. Liquid in glass remains in use where simplicity matters more than anything else.

The applications follow from range and toughness rather than from precision, since these devices are less accurate than several alternatives. Kilns, furnaces, ovens and engines are all monitored with them. Gas appliances use one as a safety device, held hot by the pilot flame and generating just enough current to hold a gas valve open, so the valve closes automatically if the flame goes out, which is elegant because it needs no power and fails safe. Food probes use them. Aerospace and power generation use them in large numbers. Spacecraft have used arrays of the same principle in reverse to generate electricity from the heat of a radioactive source, which is what powers probes travelling too far from the sun for solar panels.

Two different metals joined at one end produce a voltage that depends on the temperature difference between that junction and the free ends, and the relationship is well characterised for standard pairs. The range, speed, toughness and low cost make it the industrial default. Knowing the hot temperature requires knowing the cold one, which instruments handle by measuring their own terminals. A gas pilot flame holding one hot is what keeps a safety valve open.