The operation of a permanent magnet moving coil type instrument is based upon the principle that when a current carrying conductor is placed in a magnetic field, it is actuated upon by a force which tends to move it to one side and Out of the field. This is similar to the principle of D Arsonval galvanometer. The difference being that a direct reading instrument is provided with a pointer and a scale.
The circuit arrangement shows a series-types ohmmeter. It consists of a PMMC instrument connected in parallel with a shunting resistor R2. This parallel circuit is in series with resistor R1 and a battery of e.m.f. E. The series circuit is connected to the terminals A and B of the unknown resistance Rx.
From the fig. R1= Current limiting resistor R2 = Zero adjusting resistor Rm = Internal resistance of the meter E = e.m.f of internal Battery.
It is observed that when the unknown resistance Rx = 0 (terminals A & B shorted) maximum current flows through the meter. Under this condition resistor R2 is adjusted until the basic meter indicates full scale current Ifs. The full scale current position of the pointer is marked “0 Ω“ on the scale. Similarly when Rx is removed from the circuit i.e. Rx = , when A & B are open, the current in the meter drops to zero and meter indicates zero current which is then marked. The scale is designed to read resistance in ohms.
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0-200A with internal shunts and 0-5,000A with external shunts D.C. Voltmeter 0-100mVV without series resistance and up to 20kV or 30kV with external series resistance. Sensitivity The current sensitivity is defined as the deflection per unit current and this is equal to the ratio of constant G of electrical system divided by the control spring constant K. Thus for high sensitivity, G should be large and K should be small. For a given coil area and a constant flux density, G can be increased when the coil is would with many turns of thin wire while K can be made small by using a light flat spring and the coil assembly lightly pivoted.
On the other hand a less sensitive instrument is wound with few turns of thick wire and has a stiffer spring. Thus as a general rule it may he stated that a sensitive instrument will have a large resistance because it is wound with may turns of fine wire.
On the basis of comparison between instruments of different sensitivities, the sensitivity of the instrument may be defined s the current necessary to give full scale deflection. Modern moving coil instruments has sensitivity in the range from 25µA to 10mA. Voltmeter Sensitivity We know that PMMC movement requires series resistance of different values to give different ranges. For example a movement requiring 5mA for full scale deflection can be converted in to voltmeter of range 250V, 25V and 2.5V by connecting series resistances of different values so as to make the total resistance of 50,000 Ω, 5,000 Ω and 500 Ω respectively. In either case if we divide the number of ohms by the scale range i.e. 50.000/250 or 5.000/25 or 500/2.5. We have same quotient in ohms per Volt i.e. 200 Ω/V. This is called the sensitivity of the Voltmeter. The current drawn at full scale is the reciprocal of this number i.e. 5mA or 0.005A. Loading effect of an Instrument The sensitivity of a d.c. Voltmeter is an important factor in selection of instrument for measurement of a certain voltage. A low sensitive meter may give correct readings when used for measurement of voltage in Iow resistance circuits but will certainly give error in high resistance circuits. A voltmeter when connected across two points in a highly resistive circuit, behaves as a shunt for the portion of the circuit and, therefore, reduces the equivalent resistance in the portion of the circuit and the instrument gives lower indications of the voltage drops than actual drops before connecting the instrument. This effect is called the loading effect of an instrument. This effect is mainly with low sensitivity instruments.
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