Calibration is a comparison between two measurements of the same physical quantity - one with an instrument of known correctness, called standard instrument and another measurement made in a similar way with a second device (i.e., the device being calibrated). 
It is pertinent to mention that the standard instruments are those which are "traceable" to the internationally defined measurement units. Establishing "traceability" is accomplished by a formal comparison to a standard which is directly or indirectly related to international or national standards (e.g., National Physical Laboratory in the India). Hence, the term "measurement traceability" is used to refer to a comparison relating an instrument's measurements to a known standard.
In the present experiment, two indicating instruments are calibrated- i) an ammeter ii) a voltmeter @Calibration of Ammeter: @For calibration of an ammeter, two ammeters- standard and the test are connected in series so that same current flows through them when they are given a power supply. The schematic of the experimental setup is shown in the Fig. 1 (a). As evident from the figure, series resistances R<sub>1</sub> and R<sub>2</sub> are placed in the circuit so that for a constant D.C supply the current through the ammeters can be varied over the full scale range of the test ammeter.By adjusting R<sub>1</sub> and R<sub>2</sub> the current through the test ammeter is to be set at desired values according to the graduations or markings on the its scale and the corresponding readings of the standard ammeter are noted. (For coarse control of the current one can use R<sub>1</sub> which is nothing but a load box; and for fine adjustment of the current R<sub>2</sub> can be used which is a rheostat). From these readings one can calculate the error in the measurement by the test ammeter. It is to be noted here that, the scale of the standard ammeter should be chosen higher than that of the test ammeter.  
@Calibration of Voltmeter: @
For calibration of a voltmeter, two voltmeters- standard and the test are connected in parallel so that same potential drop appears across them when they are excited with a power supply. The schematic of the experimental setup is shown in the Fig. 1 (b). As evident from the figure that a rheostat R, connected across a constant D.C source, acts as a potential divider. Hence, by varying the moving contact of the rheostat the voltage across the two voltmeters can be varied over the full scale range of the test voltmeter.  Thus by adjusting R the voltage across the test voltmeter is to be set at desired values according to the graduations or markings on the its scale and the corresponding readings of the standard voltmeter are noted. From these readings one can calculate the error in the measurement by the test voltmeter. It is to be noted here again that, the scale of the standard voltmeter should be chosen higher than that of the test voltmeter.