True RMS Reading Voltmeter

True RMS Reading Voltmeter

The scale of general voltmeters (average reading voltmeters) is calibrated in terms of the RMS value of sinusoidal waveform only. So to read the RMS value of any waveform we use True RMS Reading Voltmeter.

In this The input signal is amplified by an AC amplifier and then the electrical energy is converted in heat energy and the temperature change is measured by the main thermal couple and hence the heat energy is again converted into electrical energy and it is then measured by the PMMC meter. Here a Balancing thermocouple is used to remove the non-linear behavior of the main thermocouple.

Here the electrical energy is converted into heat energy E and we know heat energy is proportional to the square of RMS current and voltage hence the value of RMS voltage is measured by.

Electrical energy -> Heat Energy -> Electrical Energy

Here The DC amplifier is used to amplify the potential difference created by the thermocouple.

Note: thermocouple is a device used to measure the temperature in this in this there is a potential difference is devoloped in the two rods of thermocouple according to the temperature (the two rods are of different material)

Note: The scale of the PMMC meter can be calibrated in terms of the RMS input voltage of any waveform.

Advantage: RMS voltage of any waveform can be measured.

Disadvantage: Uneconomical, chances of error are more due to many parts are present. and Energy loss.

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What is PMMC Instrument?

What is PMMC Instrument?

There are different types of electrical machines which we use in our daily life. Whenever these machines are not working properly then we have to check what is the problem for this purpose sometimes or normally we need to measure some parameters so for this we need some instruments to measure them so then PMMC ( permanent magnet moving coil ) is one of those instruments.

• PMMC  stands for "permanent magnet moving coil".
• It is a simple and frequently used instrument on ships with sophisticated names.
• Used where exact measurement is required also as an aid while maintaining electrical equipment.
• also known as D'alvanometer. because it is a kind of galvanometer that works on the principle of D'Arsonval
• These instruments use permanent magnets to create a stationary magnetic field in the coils.
• Then it is used with the moving coil that is connected to the electric source for generating deflection torque according to the Fleming left-hand rule.

Working of PMMC Instrument

• Working Principle of PMMC Instrument is simple whenever there is a flow of current in the moving coil inside the stationary magnetic field by permanent magnets there is a force there is a deflection torque generated because we know a magnetic field exerts a force on current-carrying wire inside the magnetic field according to the fleming's left-hand rule. thus there is a deflection torque is produced and we providing a damping force by the spring for maintaining the pointer at equilibrium on desired reading.

Construction of PMMC Instrument

The important parts of PMMC are as follows;

Moving Coil

•  It is an essential component of the PMMC instrument. This designing of the coil can be done by wounding copper coils on a rectangular block among the magnetic poles.
•  The rectangular block is made up of Aluminium and can be called Alumnim former rotted into the jeweled bearing. 
•  So it permits the coil to turn freely.
•  Once the current is supplied throughout these coils then deflection takes place within the stationary magnetic field and according to the deflection, we measure the voltage or current magnitude. 

Note: The aluminum is a non - metallic former, used to measure the current, and metallic former including high electromagnetic damping is used to calculate the voltage.

Magnet System

•  It includes two high-intensity magnets otherwise a 'U' Shaped magnet-based design.
•  The designing of these magnets can be done with Alnico and Alcomax for higher superior field intensity and coercive force.
• In several designs, an extra soft iron cylinder can be arranged among the magnetic poles to create the field identical; while decreasing air reluctance for increasing the strength of the field.

Control

• The deflection of the pointer is controlled by the control springs which provide equal opposite torque to balance the pointer in equilibrium position.
• Springs are fabricated by phosphorous bronze. These springs are arranged among the two jewel bearings.
• The spring provides the lane to the lead current to supply in and out of the moving coil. 
• The torque can be controlled mainly due to the delay of the ribbon.

Damping Torque

• generated by using aluminum cor's movement within the magnetic field.
• Because of the movement of coil within the magnetic field eddy currents can be generated within the aluminum former. this generates damping force otherwise to torque to resist the motion of the coil.
• So the deflection of the pointer reduces gradually and the pointer lasts at a permanent position which helps to take the right measurements.

Pointer and Scale

•  In this instrument, the connection of the pointer is done throughout the moving coil.
• So the movement of the coil results in the movement of the pointer and hence which gives the reading on the scale.
• the pointer is made up of a light material so it can easily move with the moving coil.
• Sometimes there is a chance of parallax error which is decreased by properly arranging the pointer on the blade.

Sources of Error in PMMC

• temperature effects
• getting old of the instrument cause error to the main part of the instrument like a magnet, moving coil, and spring.
• errors can be reduced by connecting swamping resistance in series using moving coil. here swamping resistance is nothing but a resistance with less temperature coefficient which reduces temperature effects on the instrument.

Torque Equation

The equation involved in the PMMC instruments is the torque equation.

                                              Td = NBLdl

where,

'N' is the no. of turns in the coil

'B' is the density of flux within the air gap

'L' and 'd' are vertical as well as horizontal lengths of the surface

'I' is the flow of current in the coil

                                       G = NBLd

The restoring torque can be provided to the moving coil is done by the spring which we say,

                                        Tc = Kθ (‘K’ is the spring constant)

The Final deflection cab be done through the equation Tc = Td

Substitute the values of Tc and Td in the above equation, then we can get

                                        Kθ = NBLdl

we know  G = NBLd

Kθ = Gl

θ= Gl/K

I = (K/G) θ

So we conclude that the deflection torque is directly proportional to the flow of current in the coil in PMMC 

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Performance Characteristics of Digital meters

• Resolution.
• Accuracy 
• Linearity - an equal amount of change in input in the same proportion there will be a change in the output.
• Settling time
• Temperature Sensitivity
• Monotonicity 

 Note: 99% are Digital Voltmeters

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Advantages and Disadvantages of Electronic Instruments

Advantages of Electronic Instruments

• Low power Consumption
• High-Frequency Range
• Better Resolution
• Storage facility
• Accuracy is high

Disadvantages of Electronic Instruments.

• Sensitivity
• effect of noise is more dominating than analog instruments. (unwanted signals are called noise)
• electronic instruments/devices are prone to damage due to the loading effect.
• Digital instruments may lose reliability.

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Wien's Bridge

 Wien's Bridge

It is primarily known as the frequency-determining bridge And is described here not only its use in ac bridges but it also has used in various other useful circuits.

The Wien's Bridge may also be employed in a harmonic distortion analyzer, where it is used as a notch filter, discriminating against one specific frequency.

It has also application in audio and HF oscillators as the frequency-determining device.

from fig at balance we have:

( R1/( 1 jwC1R1 )).R4 = (R2 - j/wC2).R3

Solving and Equating real and imaginary parts we get

and    wC1R2 - 1/wC2R1 = 0 from which w = 1/(R1.R2.C1.C2)^(1/2)

and frequency  

 In most Wien's bridge , the components are so chosen that 

                 R1 = R2 = R          and    C1 = C2 = C

          THe eq reduces to : R4/R3 = 2 

and              f = 1/(2.pi.R.C)

Switch resistors R1 and R2 are mechanically liked so as to fulfill the condition R1 = R2

As long as C1 = C2  are fixed capacitors and R4 = 2R3, the Wien's bridge may be used as a frequency-determining device, balanced by a single control. This control may be directly calibrated in terms of frequency.

This bridge is suitable for the measurement of frequency in the range of 100Hz to 100KHz  with an accuracy of 0.1 to 0.5 percent.

Because of frequency sensitivity, The Wien's bridge is difficult to balance unless the waveform is perfectly sinusoidal in nature.

The bridge is not balanced for any harmonics present in the applied voltage, Thus this harmonics will sometimes produce an output voltage masking the true balance point. This difficulty can be overcome by connecting a filter in series with a null detector.

Wien's bridge may be used for the measurement of capacitance also.

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Campbell's Bridge

 Campbell's Bridge

This bridge measures unknown mutual inductance in terms of standard mutual inductance

Let   M1 = unknown mutual inductance,

         L1 = self-inductance of secondary of mutual inductance M1,

         M2 = variable standard mutual inductance,

          L2 = self-inductance of secondary of mutual inductance M2,

and   R1,R2,R3,R4 = non - inductive resistances.

There are two steps in balancing the process.

1.    Detacr is connected between b and d and the balanced point is obtained then the requirement of balance is :

                            L1/L2 = R1/R2 = R3/R4

This bridge may be balanced by adjustment of R3 ( or R4) and R1 ( or R2).

2.    Then we connected between b' and d' . Keeping adjustment in step1  then variable mutual inductance M2 is varied to get balance point Then 

                            M1/M2 = R3/R4

                            M1 = M2.( R3/R4 )

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Campbell's Modification of Heaviside Bridge

 Campbell's Modification of Heaviside Bridge

fig shows a modified Heaviside bridge. This modification is due to Campbell. This is used to measure self-inductance. In this case an additional balancing coil L, R is included in arm ad in series with inductor under test. An additional resistance r is put in arm ab . Balance is obtained by varying M and r. A short-circuiting switch is placed across the coil R2, L2 under measurement. Two sets of readings are taken one with the switch being open and the coil R2, L2 under measurement. Two sets of readings are taken one with the switch being open and the other with the switch being closed. Let values of M and r, Be M1 and R1 with the switch open and M1 and r2 with the switch closed.

we have L2+L = ( M(R3 + R4) + R4.L1 )/R3

and               L = ( M2 ( R3 + R4) + R4.L1)/R3

  Therfore     L2 = (M1 - M2)(1 + R4/R3)

similarly  we can write: R2+R = (R1 + r1).R4/R3  and R = (R1 + R2).R4/R3

                       R2= (r1 - r2).R4/R3

This method is a good example of the method adopted to eliminate the effects of leads etc.

if R3 = R4

            L2 = 2(M1 - M2)

            R2 =  r1 - r2

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Heaviside Mutual Inductance Bridge.

  Heaviside Mutual Inductance Bridge.

This bridge uses known self-inductance to measure mutual inductance. The same bridge slightly modified by Campbell is used to measure self-inductance in terms of known mutual inductance.

M = unknown mutual inductance,

L1 = self-inductance of secondary of mutual inductance,

L2 = known self-inductance,

and R1 , R2 ,R3 , R4 = non-inductive resistors.

At balance voltage drop across between b and c must equal to the voltage drop across between d and c. Also, the voltage drop across ahc must equal to the voltage drop across adc. Thus we have the following equations at balance.

                    I1R3 = I2.R4

               (I1+I2)(jwM)+I1(R1+R3+jwL1) = I2(R2+R4+jwL2)

Solving them we get:

                R1.R4 = R2.R3

and           M = (R3.L2 - R4.L1)/(R4+R3)

if R3 = R4    ,  we get,  M = (L2 - L1)/2

                           and     R1 = R2

This method can also ussed for measurement of self inductance. Suppose if we have to determine L2 then

 we can write ,                L2 = (M(R3+R4) + R4.L1 )/R3

                                             = M ( 1 + R4/R3) + ( R4/R3 ).L1

                        and R2 = R1.(R4/R3)

In case             R4 = R3, we have

                        L2 = 2M +L1

 and                 R2 = R1 

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Anderson's Bridge

 Introduction

This Bridge, in fact, is a modification of maxwell's inductance capacitance bridge in this method we measure unknown self-inductance in terms of a standard capacitor this method is applicable for the precise measurement of self-inductance over a wide range of values.

Let L1=Self inductance be measured,

      r1= resistance connected in series with-inductor,

r, R2, R3, R4= known non-inductive resistances,

R1= resistance of the self inductor,

C= fixed standard capacitor.

At balance , I1=I3 and I2 = Ic + I4

Writing and solving balance equations gives us the result 

and

Two obtain easy convergence of balance alternate adjustments of r1 and r are done.

Advantages:

• for a low-value Q coil it is easy to obtain a balance point because there is no sliding occurs like maxwell's bridge.
•  A fixed capacitor is used instead of a variable capacitor like in maxwell's bridge.
• This bridge may be used for accurate measurement of capacitance in terms of inductance

Disadvantages:

• The bridge is more complex than maxwell's bridge, due to more parts it is difficult to manipulate the bridge for different measurements and also to set up . The balance equations are not simple and in fact tedious.

Considering the above complications in the cases where a variable capacitor is permissible maxwell's bridge is used.

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Hay's Bridge

  Hay's Bridge

It is the modification of maxwell's bridge. This bridge uses resistance in series with the standard capacitor.

(unlike the Maxwell's Bridge which uses resistance in parallel with the capacitor).

Let L1 = unknown inductance having resistance R1,

R2, R3, R4= known non-inductive resistances, and C4=standard capacitor.

At balance,

  (R1+jwL1)(R4-j/wC4)=R2.R3

Separating real and imaginary terms and solving we get :

                L1=(R2.R3.C4)/(1+w^2.C^2.R4^2)

and          R1= (w^2.R2.R3.R4.C4^2)/(1+w^2.C4^2.R4^2)

The Q factor of the coil is : Q= (wL1)/R1 = 1/(wC4R4)

there is a frequency term in the above expressions so it seems that it must be accurately known. This is not true for inductance when a high Q coil is being measured. Because:

we can write 

L1= (R2.R3.C4)/(1+(1/Q)^2)

for Q value higher than 10  the equation reduces to L1= R2.R3.C4

we can see the eq. becomes the same as maxwell's bridge.

Advantages:

• This bridge gives a very simple expression for unknown inductance for high Q coils and is suitable for coils having Q>10.
• This Bridge gives a simple expression for the Q factor.
• If we examine the expression for Q factor: Q = 1/(w.C4.R4)

R4 appears in denominator means for a high value of Q we have a small value of R4 thus bridge requires a low value of R4 whereas The Maxwell's bridge requires a parallel resistor, R4 of a very high value.

Disadvantages:

• The bridge is not suitable for the measurement of inductance Q <10.

Reference:

Swerny 

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Measurement of Self Inductance , Maxwell's Bridge

Maxwell's Inductance Bridge: The bridge circuit measures unknown inductance by comparison with a variable standard self-inductance.

Let L1=unknown inductance of resistance R1,

      L2=variable inductance of fixed resistance r2,

      R2= variable resistance connected in series with inductor L2,

      R3, R4= known non-inductive resistances.

By solving balance equations of the bridge we get

                          L1=(R3/R4).L2

                          R1=(R3/R4)(R2+r2)

Resistors R3 and R4 normally a selection of values from 10,100,1000,10,000ohm ,r2 is a decade resistance box. In some cases, an additionally known resistance may have to be inserted in series with an unknown coil in order to obtain balance.

Maxwell's Inductance-Capacitance Bridge:

In this bridge, inductance is measured by comparison with a variable standard capacitance.

Let L1=unknown inductance,   R1= effective resistance of inductor L1,

      R2, R3, R4=known non-inductive resistances, and C4=variable standard capacitor.

 

Writing an equation for balance

(R1+jwL1)(R4/(1+jwC4R4))=R2.R3

Separating the real and imaginary parts and solving we get:

                                R1=(R2.R3)/R4

                        and  L1=R2.R3.C4

  the two variables R4 and C4 appear in different equations so the equations are independent.

The expression for Q factor Q= (w.L1)/R1 = w.C4.R4

Advantages 

• The two balance equations are independent if we choose R4 and C4 as variable elements.
• The frequency does not appear in any of the two equations.
• This equation gives a simple expression for unknowns L1 and R1 in terms of bridge elements.
• for example, if we consider R2.R3=10^6 then 
•                                L1=C4.10^6
• thus if balance is achieved then if the value of capacitance is in micrometer then it gives the direct value of inductance in henry.
• Maxwell's inductance-capacitance bridge is very useful for the measurement of a wide range of inductance at power and audio frequencies.

Disadvantages

1.    The variable capacitor is very expensive if calibrated to a high degree of accuracy

Therefore sometimes a fixed capacitor is used which is known for a high degree of accuracy. In this case, balance is obtained by

(a)   either varying R2 and R4 since R2 appears in both equations thus balance adjustment becomes difficult.

(b) putting additional series resistance with inductance and varying this and R4.

2.    The bridge is limited to measurement of low Q (1<Q<10) .Because for high Q value of R4 must be high as 10^5 or 10^6 and the resistance box with such high resistance is very expensive. Thus bridge is unsuitable for Q>10.

The bridge is also unsuitable for low Q (Q<1) . Because for low Q obtaining balance condition is difficult due to the sliding effect.

 ( The capacitance is generally fixed in the measurement of balance point thus we change values of R2 and R4 and this comes in both equations so if we change one then another variable changes and balance disturbs so we need to do many manipulations) 

Thus we see from the above discussion Maxwell's bridge is suitable for the measurement of medium Q values only.

Reference:

-Swerny

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AC bridge Sources and Detectors

 AC bridge Sources and Detectors

Measurement at low frequency: power line may act as a source of supply to the bridge circuits.

Measurement at higher frequency: Electronic oscillators are universally used as bridge source supplies.

Electronic oscillators and higher frequency

1. Frequency is constant, easily adjustable, and determinable with accuracy.

2. The waveform is very close to a sine wave.

3. Their power output is sufficient for most bridge measurements 

(A typical oscillator has a frequency range of 40Hz to 125Hz with a power output of 7 W.)

Detectors

Commonly used detectors for ac bridges are Headphones, Vibration galvanometers, and Tuneable amplifier detectors. 

Headphones are widely used as detectors at frequencies of 250Hz and over up to 3 or 4kHz.They are the most sensitive detectors for this frequency range.

At a single frequency: Tuned detectors normally give the greatest sensitivity and discrimination against harmonics in the supply.

Vibration galvanometers are extremely useful at power and low audion\ frequency ranges. Vibration galvanometers are manufactured to work at various frequencies ranging from 5Hz to 1000Hz but are most commonly used below 200Hz as below this frequency they are more sensitive the headphones.

Tuneable amplifier detectors are the most versatile of the detectors.

The transistor amplifier can be tuned electrically and thus can be made to respond to a pointer-type instrument.

This detector can be used, over a frequency range of 10 Hz to 100KHz

For ordinary a.c.bridge measurements of Inductance and Capacitance.

 A fixed frequency oscillator of 1000Hz and output of about 1W is adequate. But for more specialized work variable oscillators are preferable with output up to 5W. In practice power is supplied is within limits to the bridge but on some occasions, a higher power may be necessary. 

Usually, an untuned amplifier is used as a detector. The balance detection is sensed both orally by headphones and visually by a pointer galvanometer having a logarithmic deflection (to avoid damage to the galvanometer which may be caused by unbalancing).

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Laboratory Standards of Emf(Zener controlled voltage sources)

In the past standard, cells were used exclusively as laboratory standards but in recent years. Semiconductor devices such as Zener-controlled reference sources have replaced standard cells in practically all industrial applications.

Silicon diodes have voltage-current characteristics such that an extremely sharp reverse current occurs at a point on the voltage curve known as "Zener voltage". 

The point indicates a breakdown of the diode under reverse voltage application, but the process is reversible if safe current and heating limits are not exceeded. 

The Zener voltage may be controlled over a wide range by the processing techniques used during the manufacture of the Zener diodes.

In the fig, the circuit is of Zener diode in a typical application as a laboratory standard.

• he supply voltage E is usually much higher than the Zener voltage of the diode assuming the breakdown of the diode.
• resistor R usually has a high value and is placed in series with the diode and the battery. It serves to limit the current through the diode during breakdown to a safe value.
• In addition, when the supply voltage varies for any reason, most of this variation is taken up in the voltage drop across R with a very small change in diode current and as a result, the reference voltage ( output voltage) remains practically the same.

        

Commercial voltage standards using Zener diodes essentially consist of :

(i)    A Zener controlled voltage source placed in a temperature-controlled environment to improve its long-term stability.

(ii)    A precision output voltage divider. The temperature is controlled to +0.03 degree Celcius over an ambient temperature range of 0 degree Celcius to 50 degree Celcius providing output stability of the order of 10 ppm/month.

The four available outputs are :

(a) a 0 -1000 microVolt source with 1 microVolt precision,

(b) a 1.00 V reference for potentiometer measurements with ratio box,

(c) a 1.018+(delta) reference for saturated Weston cells comparisons.

(d) a 1.019+(delta) reference for unsaturated Weston cell comparison.

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Secondary Standard of Emf

The secondary standard of Emf is the unsaturated Weston cell.

Required Posts

Saturated Weston Cell (Primary Standard of Emf )

Unsaturated Weston cell 

Construction and Characteristics. The unsaturated cell is similar to a suturated cell except that it does not have Cadmium sulphate crystals ( CdSO4 ).

• It has the same electrodes but the solution is saturated at 4 degree Cecius , and therefore unsaturated at room temperature.
• In addition, a 'sputum' or retaining member (porous plugs in this case ) is used over each electrode to hold the material in place, and hence these cells are considered portable.
• After being moved from one place to another, the cell should be allowed a few days to settle so that the results obtained from measurements involving cells are accurate.
• The cells are enclosed in a bakelite case which is lined with 1.5mm thick copper to help keep all parts of the cell at a uniform temperature.
• Not all unsaturated cells have the same emf. In fact, saturated cells differ from one another.
• The emf of new unsaturated cells lies between 1.0190 to 1.094 absolute volt. Therefore each cell must be calibrated against the primary standard in order to know its exact emf. 
• The temperature at which this emf is measured should also be specified.

Note: The unsaturated cell becomes less stable with age, yet it is remarkably good. The decrease in emf is about 30 to 50 microVolt per year. It is recommended that the cells be checked by returning them to a standardizing laboratory once a year for certification.

• An unsaturated cell has a shorter life than a saturated type but some cells last even 20years. age of the cell life also depends on the kind of treatment the cell received.

The temperature co-efficient: it is less than that of a saturated cell. It is -10microVolt per degree Celcius. Thus can be used for most purposes without temperature correction., if the temperature is near to the calibration temperature.

The cell has a positive temperature effect on one electrode and a negative at the other. Therefore it is important to keep all the parts at the same temperature.

Precautions. 

• Same precautions are followed as saturated cells except for some difference that is cel is portable and can be shipped.
• cell should be not operated below 4 degree Celcius and below 40 degree Celcius 

Note: Below 4 degree Celcius the emf drops rapidly and eventually the electrolyte freezes. Above 40 degree Celcius the cell materials start to melt above voltage drops sharply around 55 degree Celcius . It has been mentioned earlier that unsaturated cells lose emf, at an average rate of 30-50 microVolt per year at normal temperature of use and storage. At higher temperatures, the decrease is much more rapid and is about 240 microVolt per year 60 degree Celcius.

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Primary Standard of Emf (Saturated normal Weston cell )

The saturated normal Weston cell is known as the primary standard voltage source developed by Edward Weston in 1892. 

Note: this is a voltaic cell.

Construction and Characteristics: the construction shown in figure

Positive electrode: Mercury

Negative electrode: An amalgam of cadmium which is 1 part of cadmium and 7 parts of mercury.

Electrolyte: Saturated solution of cadmium sulphate.

Depolarizing agent: A paste of mercurous sulphate Hg2So4 is placed on top of mercury.

Note: An excess of solid crystals of CdSO4.8H2O ( cadmium sulphate) in the electrolyte space and the depolarizer assures that the solution will be saturated at all temperatures.

A simplified cell reaction is : 

-The emf of the cell remains constant as long as amalgam has both solid and liquid phases are present.

-The Weston cell is contained in an H- Shaped vessel. 

-The limbs of the H vessel are hermetically sealed and the connections to the external circuit are made through platinum wires.

-Sulphuric acid is added to act as an electrolyte.

-Saturated cells are normally allowed to age for a year before they are considered to be of constant emf. Following this period, their life appears indefinite excluding the possibility of accidents.

The emf of the cell is 1.01864 volt.

The emf of the cell changes with temperature and the equation relating emf to temperature is :

 where Et = emf of the cell at any temperature t degree Celcius,and E20 = emf of the cell at 20 degree Celcius

Note: The cell voltage drops by about 40 microvolts per degree Celcius. This cell is kept in an oil bath to control its temperature within 0.01 degree Celcius.

The cell has a long life if handled carefully. It has a life span of about 10 to 20 years. The drift in value of voltage is about 1 microvolt per year.

Note: The cell is a standard for maintenance of the volt and as such, it is only used in standardizing laboratories. Saturated cells are not used for ordinary work because of temperature requirements and also because they are not portable. The unsaturated cell is used for ordinary measurments.

Precautions:

    1. Mercurous sulphate is sensitive to light and therefore it should not be exposed to light .

Note: Exposure to an incandescent light at ordinary levels of illumination is much less serious than direct sunlight or even light from the usual fluorescent lamps.

    2. It has been mentioned above that the cell has a temperature coefficient of about -40 microvolts per degree Celcius. In that, it has an effect of +310 microvolts per degree Celcius at the positive terminal and -350 microvolts per degree Celcius at the negative terminal. 

Therefore it is important to keep the entire cell at a uniform temperature to secure as much cancellation of positive and negative effects. Hence these cells always used in close temperature regulation, usually an oil bath held constant to better than 0.01 degree Celcius.

    3. Saturated cells should be kept in an upright position because if they are tilted there is always the danger of mixing of electrode materials. Therefore these cells cannot be shipped and are sent through special messengers.

    4. The principal limitation of the use of a standard cell is the current drawn from the cell at any time. This should be negligibly small. Even this small current should be allowed to flow for a very short period of time. It should be understood that standard cells are potential devices and will be damaged if the current is drawn from them.

it is difficult to measure the maximum value of current drawn from a standard cell since damage to the cell is a function of both magnitude and duration of current flow. The makers specify the maximum value as 100 microamperes but this should be regarded as an extreme figure. This means that the current drawn from the cell should be less than 100 microamperes and this current should flow momentarily. 

A voltmeter should never be used for measuring the voltage of a standard cell. Firstly because of the current drain, there is a possibility of damage to the cell and secondly, this measurement is meaningless since the cells have a high internal resistance ( about 600 to 800 ). The voltage should be measured with the help of a potentiometer.

A cell should be handled with care and never be short-circuited. The excessive current causes a change in emf which may be permanent because the cell may not be able to recover. A cell once short-circuited, should be regarded with suspicion and hence is of little value as a standard. 

    When using the cell in potentiometric work, a resistance of 20,000 ohm should be inserted in series with the cell in order to avoid to an excessive drain of current when there is unbalance.

It is important that leakage resistance between the terminals of the cell be very high otherwise it may not give the standard voltage it is supposed to give.

Example1: A standard cell has a voltage rating of 1.018500 V and internal resistance of 500 ohm. The insulation resistance between its terminals is 5 Mohm

. Find the current drain due to insulation resistance. Calculate also the difference between the internal voltage and the terminal voltage.

Solution: Current flowing through the insulation resistance =   (( 1.018500 )/(500+5x10^6 ) ) =0.204x10^-6 A

=0.204 micro Ampere

Thus there is a constant drain of a current of 0.204 micro Ampere.

Voltage drop = ( 0.204 x 10^-6 ) x 500

                     = 0.102 x 10^-3 V = 0.102 mV

   Therfore , Difference between internal voltage and terminal voltage = 0.102 mV

    Terminal voltage = 1.018500 - 0.000102

                                = 1.018398 V.

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