By exposing the hot end of the junction to successive standard temperatures, and maintaining the cold ends at a known constant temperature, the necessary data for inclusion in a formula may be obtained.
In fixing a permanent temperature scale, calculated from the formula, to a millivoltmeter, it must be remembered that the values given by the experiment are absolute, and independent of the resistance of the circuit composed of the thermo-element and galvanometer. On the other hand, a millivoltmeter is marked to read difference of potential at its terminals; and if in series with a junction and leads of notable resistance, its indications will not be the E.M.F. of the junction. An example will make this point clear.
Example.—A millivoltmeter has a resistance of 100 ohms, and is marked to read P.D. at its terminals. A thermocouple and leads connected to the millivoltmeter have a resistance of 5 ohms. To find the relation between the true E.M.F. of the junction and the readings of the indicator.
If E = the E.M.F. developed by the junction, and V, the reading of the millivoltmeter, = P.D. at its terminals, then the current in the circuit = E⁄105 = V⁄100; and V = (100⁄105)E. That is, the readings are lower by 5 per cent. than the true E.M.F. of the junction. In the same way a low resistance voltmeter, if applied to a cell of high resistance, shows a lower reading than the E.M.F. of the cell.
This example indicates how a table connecting true E.M.F.’s with reading in millivolts may be calculated when the resistances concerned are known. It is presumed, in preparing a scale in this manner, that the resistance of the couple will not be subject to such alterations as to affect the reading.
The advantages of this method of calibration are manifest when a number of junctions are being made from a given batch of wires, as it is only necessary to divide the scale of the indicator so as to represent millivolts—a simple operation—and then to attach a temperature scale. This procedure is much more expeditious than standardizing each indicator at several fixed points when a number are concerned, but for a single junction the fixed point method is easier. The potentiometer method of measuring E.M.F. may also be used to determine temperatures in place of an indicator, and is of great service in cases where very accurate readings are specially required, being far more delicate in detecting small differences of temperature than an indicator. Special potentiometers for thermo-electric work are made by the Cambridge and Paul Instrument Company, Siemens, and others, and are useful in conducting accurate research, but are too elaborate for workshop or ordinary laboratory practice.
Cold Junction Compensators.—The necessity for paying attention to the cold junction has led to various attempts to compensate automatically for changes of temperature at this part of the pyrometer. A thermometer located near the cold junction, as in [fig. 6], is all that is needed to correct a two-junction circuit; but when a three-junction circuit is used a correct reading is not secured by adding the excess temperature of the thermometer over the calibration temperature to the reading on the indicator. In Bristol’s arrangement a mercury thermometer, with a large bulb and wide stem, is stationed at the cold junction, and participates in any temperature change. In the stem is placed a loop of thin platinum wire, which forms part of the pyrometer circuit. When the mercury is heated it expands up the stem and short-circuits a portion of the loop, thereby diminishing the resistance of the pyrometer circuit, and tending to increase the deflection on the indicator. Simultaneously the cold junction will be heated, tending to diminish the current, and so to cause a less deflection. By adjustment these two tendencies may be counterbalanced, so that the reading is unaffected, but such adjustment will only apply to a given E.M.F., and therefore to one temperature of the hot junction. Hence this method fails in general application.
Peake’s compensated leads are intended to remedy cold-junction errors by transferring this junction, in effect, to the galvanometer. They are used for pyrometers in which the platinum metals are employed, and consist of wires of two different alloys of copper and nickel, which connect the cold end to the indicator. These alloys are such that the electromotive forces set up at the junctions in the head—Pt and Cu-Ni 1, and Pt-Ir with Cu-Ni 2—are equal and opposite at all working temperatures, and hence changes at the cold junctions do not affect the reading. At the indicator, however, temperature changes would cause an alteration in deflection; but as the indicator is generally placed well away from the furnace, and is not liable to notable heating or cooling, the possible errors are greatly reduced by the use of these leads. They are obviously of no value for use with base-metal pyrometers, as the wires used in such may be prolonged to the indicator, with an identical result.
