The basic principle of thermoelectric thermometry is that a thermocouple develops an emf which is a function of the difference in temperature of its measuring junction and reference junction. If the temperature of the reference junction is known, the temperature of the measuring junction can be determined by measuring the emf generated in the circuit. The use of a thermocouple in temperature measurements, therefore, requires the use of an instrument capable of measuring emf. There are two types of emf measuring instruments in use in industry-deflection meters (millivoltmeters) and potentiometers. The digital voltmeter, which is now being used extensively, is a type of potentiometer. Because of its limitations, the deflection meter is not used for precise measurements. The deflection meter consists of a galvanometer with a rigid pointer which moves over a scale graduated in millivolts or degrees. The galvanometer indicates by its deflection the magnitude of the current passing through it, and if the circuit in which it is placed includes a thermocouple, it measures the current I generated by the thermocouple in the circuit. If the circuit has a resistance R and the emf is E, by Ohm's law, E = RI. If R is kept constant, I is proportional to E, then the scale can be calibrated in terms of millivolts rather than in milli- or microamperes. This calibration holds as long as R remains constant. Any change in R introduces an error in the indicated value of E. Changes in resistance may result from changes in temperature of the thermocouple or its extension wires or of the copper galvanometer coil, from corrosion of the thermocouple wires, from changes in the depth of immersion of the thermocouple, or from changes in contact resistance at switches or binding posts.