_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.