Absolute Instruments and Secondary Instruments

Absolute instruments and secondary instruments are the two basic classifications of measuring instruments.

You may be wondering why a tangent galvanometer is an absolute instrument while an ammeter is considered a secondary instrument even though both are used to measure current. The reason behind this is explained in this article.

Let’s dive in.

Absolute Instruments

Absolute instruments give the values of the quantity that has to be measured in terms of physical constants and their deflection only.

In other words, absolute instruments give the value of the measurand in terms of instrument constant and its deflection.

Such instruments do not require comparison with any other standard. They do not need any calibration or comparison with another standard instrument. They are highly accurate instruments.

In absolute instrument, the measured value is given in term of instrument constants and the deflection of one part of the instrument

Working with absolute instruments for routine work is time-consuming since every time a measurement is made, it takes a lot of time to compute the magnitude of the quantity under measurement. So they are only be used in laboratories and standard institutions.

Absolute Instrument Examples

Now let us learn the examples of absolute instruments. The following are the main three absolute instruments.

1. Tangent Galvanometer
2. Rayleigh’s Current Balance
3. Absolute Electrometer

Tangent Galvanometer

An example of an absolute instrument is the tangent galvanometer.

A tangent galvanometer gives the value of the current to be measured in terms of the tangent of the angle of deflection produced, the horizontal component of the earth’s magnetic field, the radius and the number of turns of the wire used.

Rayleigh’s Current Balance

Another absolute instrument is Rayleigh’s current balance.

The ampere balance or current balance is used for the precise measurement of the SI unit of electric current, the ampere.

The current to be measured is passed in series through two coils of wire, one of which is attached to one arm of a sensitive balance. The magnetic force between the two coils is measured by the amount of weight needed on the other arm of the balance to keep it in equilibrium. This is used to calculate the magnitude of the current.

Absolute Electrometer

Another example of absolute instrument is an Absolute Electrometer.

The electrometer is a device that is used to measure electric potentials; it is absolute when the potentials are evaluated through the direct measurement of a force and consequently no previous calibration, using a known voltage, is necessary. The absolute electrometer was first proposed by Lord Kelvin.

Secondary Instruments

Secondary instruments are so constructed that the deflection of such instruments gives the magnitude of the electrical quantity to be measured directly.

These instruments are required to be calibrated by comparison with either an absolute instrument or with another secondary instrument, which has already been calibrated before the use.

Since absolute instruments for routine work is time-consuming secondary instruments are most commonly used. Absolute instruments are seldom used except in standards institutions while secondary instruments find usage almost in every sphere of measurement.

Classification of Secondary Instruments

The secondary instrument may be classified into the following categories:

1. Indicating Instruments
2. Recording Instruments
3. Integrating Instruments

Indicating instruments

Indicating instruments are those which indicate the magnitude of an electrical quantity at the time when it is being measured.

The magnitude of quantity being measured is obtained by deflection of the pointer on the calibrated scale and the output is indicated either in analogue or digital form like ammeter, voltmeter, and wattmeter.

Examples of Indicating Instruments

Ordinary ammeters, voltmeters, wattmeters, frequency meters, power factor meters, PMMC Instrument etc., fall into this category.

Three forces were acting on the pointer to deflect these instruments in proportional to the quantity being measured, these forces are of the following types:

1. Deflecting Force: This force gives the pointer the initial force to move it from zero position, it’s also called deflecting force.
2. Controlling Force: This force control and limits the deflection of the pointer on the scale which must be proportional to the measured value, and also ensure that the deflection is always the same for the same values
3. Damping Force: This force is necessary to bring the pointer quickly to the measured value, and then stop without any oscillation. Read types of damping systems.

Recording Instruments

Recording instruments are those which keep a continuous record of the variation of the magnitude of an electrical quantity to be observed over a definite period of time.

In such instruments, the moving system carries an inked pen which touches lightly a sheet of paper wrapped over a drum moving with uniform slow motion in a direction perpendicular to that of the direction of the pointer. Thus, a curve is traced which shows the variations in the magnitude of the electrical quantity under observation over a definite period of time.

Recording Instruments Examples

The instruments like recording devices, X-Y plotter, and oscilloscope and recording instruments.

Such instruments are generally used in powerhouses where the current, voltage, power, etc., are to be maintained within a certain acceptable limit.

Integrating Instruments

Integrating instruments are those which measure the total amount of either quantity of electricity (ampere-hours) or electrical energy supplied over a period of time.

Integrating Instrument Examples:

The ampere-hour meters and energy meters fall in the class of integrating instruments

The summation, given by such an instrument, is the product of time and an electrical quantity under measurement.

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