How to Calibrate a DC Ammeter using a Potentiometer

We can calibrate a dc ammeter by connecting it in series with a precision resistor/standard resistor, and accurately measuring the resistor voltage drop. The level of ammeter current is determined by dividing the resistor voltage drop by its resistance value. When the resistor value is known precisely, and the voltage drop is measured by a potentiometer, the level of ammeter current can be determined with great accuracy.

The diagram below shows a circuit employing a potentiometer to calibrate dc ammeter:

Precision resistors are normally available in shunt boxes solely for the purpose of calibrating ammeters by means of a potentiometer. A shunt box is a set of precision series-connected resistors for use in potentiometer calibration of ammeters. A movable contact selects the point at which the resistors are connected in series with the ammeter. The circuit and terminals of a typical shunt box is shown below:

The shunt box resistors are actually connected in a circular manner around the terminals of a rotary switch. In the figure above, the resistors and the rotary switch are shown in a straight line formation. The line terminals of the shunt box are connected in series with the ammeter, and the shunt box potentiometer terminals go to the potentiometer just as illustrated in the figure above.

When the shunt box is set to multiplier 1 position, the resistances in series with the line terminals add up to 1 Ω. If for example, we assume that the measured voltage is precisely 0.1 V, the ammeter current is calculated as 0.1 V/1 Ω = 0.1 A or simply 0.1 x (multiplier 1) = 0.1 A. When the shunt box is at the multiplier 0.5 position, the total resistance is 2 Ω. The ammeter current is now calculated as (measured voltage) x 0.5. In the same way, at all other shunt box positions, the current in amperes is calculated as (measured voltage) x multiplier.

In reference to figure 2 above, the total resistance in series with the ammeter is 0.5 Ω, that is, the total of all resistors to the left of the movable contact. Note that the resistors to the right of the movable contact are actually in series with the galvanometer of the potentiometer. This brings to a total of 1.5 Ω for the contact position shown in the figure above, which is much smaller than the galvanometer resistance. Furthermore, at null there is no current flowing in the galvanometer circuit, and there is no voltage drop across the 1.5 Ω resistance. Therefore these resistors have no effect on the measurement.

For each multiplier position on the shunt box, there is a maximum current level identified as MAX AMPS. In each case, this current multiplied by the resistance in series with the line terminals gives an output (to the potentiometer) of 150 mV from the shunt box. At the multiply by 100 position, the maximum current is 15 A. Thus, the output voltage is 15 A x 0.01 Ω = 150 mV. The maximum current level is selected to minimize the resistor power dissipation and the resultant temperature errors.

Related articles:

• How to calibrate a dc ammeter
• How to calibrate a dc voltmeter
• What is Calibration of the Sensor?
• Standards Used in Measurements & Their Classifications
• Absolute vs. Secondary Instruments