Why Converting a Galvanometer to a Voltmeter Matters in Physics
Let’s suppose that the water has to be transferred from one tank to another. The energy/push given for the water supply is the potential difference and the measure of the same is done by a voltmeter.
If the waterfalls in minute drops (small electric current), then this water flow is measured by a galvanometer.
The conversion of galvanometer into voltmeter is done by adding a highly-resistive multiplier in series.
Galvanometer to Voltmeter Formula
We know that for the conversion of a galvanometer into a voltmeter, a high resistance is required. So, if the resistance of the galvanometer is G and that of the high resistance is R, when they are connected in series, the total resistance of the arrangement becomes the following:
RSeries = G + R
Now, the galvanometer behaves as a voltmeter. How does this happen? Now, we will look at the same thing in the form of the following experiment.
Galvanometer to Voltmeter Conversion Experiment
We all know that the potential difference applied across the ends of a conductor is a voltmeter. We can convert the galvanometer into a voltmeter if we know its resistance and the figure of merit.
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Now, let’s perform an experiment to determine the readings we encounter while doing the conversion of galvanometer into voltmeter:
The Objective of the Experiment:
To Convert the Given Galvanometer of Known Resistance and Figure of Merit into a Voltmeter of Desired Range and to Verify the same Experiment.
Apparatus Required for this Experiment:
The following are the instruments required to perform the conversion of galvanometer into voltmeter:
A galvanometer
Voltmeter of 0-to-3 V
A source: battery
Two one way keys
Two resistance box, one of 10,000 ohms and another of 200 ohms
Rheostat: A variable resistor
Connection wires
A sandpaper
The formula for the series resistance required for the conversion is:
R = V/Ig - G
Procedure for the Conversion of a Galvanometer into a Voltmeter:
Connect the resistance box in a series combination across the galvanometer and then collect the plugs of resistance R.
In the above diagram, A and B are the fixed ends/terminals and C is the variable terminal of the rheostat (a variable resistor).
We can see that the galvanometer works as a voltmeter under the range of V Volts.
Now, take out the plugs of calculated resistance R from the resistance box.
Now, use the key to adjust the movable contact of the rheostat such that the deflection of the galvanometer reaches the maximum ranger.
Note the readings of both the galvanometer and voltmeter.
Convert the readings of the galvanometer into V or volts.
Check if there is a difference in the reading and this difference between voltmeter reading and galvanometer reading may show an error.
Now, by moving the variable contact of a rheostat, take six readings covering the range of voltmeters from 0-to-3 V.
Observation of the above Experiment:
Conversion of Galvanometer into Voltmeter Practical Observations
Calculation part:
Here,
The resistance of the galvanometer is =
The current for a full-scale deflection is = I
Number of divisions on the given galvanometer scale is = n
The figure of merit of galvanometer formula is:
Ig = nk or k = Ig/n,is the required figure of merit of galvanometer formula.
The resistance in series to be calculated will be used in the following formula:
R = V/Ig - G
The final result of the experiment:
We found that there is a minute difference in the value of the actual and the measured one and the conversion is seemingly perfect.
Conversion of Galvanometer into Ammeter Practical Observations
For the conversion of a galvanometer into an ammeter of range ‘I’, we require a shunt resistance and it can be calculated by the following formula:
S = (Ig * G)/(I - Ig)
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Where,
S = shunt resistance
Ig = nk is the needed full-scale deflection of a galvanometer. The unit of the figure of merit of a galvanometer is amp/div.
I = A range of the desired ammeter in mA
The length of the wire required to create a shunt is calculated by:
I = π\[^{r^{2}}\] S/ρ
Where,
r = radius of the wire calculated by using a screw gauge. This wire is used for making the shunt
ρ = the resistivity of the given material wire
Conversion of Galvanometer into Ammeter Practical Observations:
The resistance of the galvanometer in …..ohms.
The figure of merit in……..ampere per division.
A number of divisions in a given galvanometer, n…..
The desired range of the current in an ammeter is…...milliamperes.
FAQs on How to Convert a Galvanometer into a Voltmeter: Step-by-Step Guide
1. What is the main principle behind converting a galvanometer into a voltmeter?
The main principle is to increase the overall resistance of the galvanometer. This is achieved by connecting a high value of resistance in series with the galvanometer's coil. This ensures the device draws a negligible amount of current from the main circuit, thereby not altering the potential difference it is intended to measure.
2. How do you convert a galvanometer into a voltmeter of a desired range?
To convert a galvanometer into a voltmeter that can measure up to a specific voltage 'V', a high resistance 'R' is connected in series with it. The value of this resistance is calculated to ensure that when the maximum voltage 'V' is applied across the combination, only the full-scale deflection current (I_g) flows through the galvanometer.
3. What is the formula used to calculate the series resistance for converting a galvanometer into a voltmeter?
The value of the high resistance (R) to be connected in series can be calculated using the formula:
R = (V / I_g) - G
Where:
- V is the maximum voltage to be measured (the range of the voltmeter).
- I_g is the current required for full-scale deflection of the galvanometer.
- G is the internal resistance of the galvanometer.
4. Why must the resistance connected to the galvanometer be high and connected in series?
The resistance must be high and connected in series for two primary reasons:
- To protect the galvanometer: A galvanometer is a sensitive device that can only handle small currents. The high series resistance limits the current flow to a safe level (I_g) even when measuring high voltages.
- To ensure accuracy: A voltmeter is always connected in parallel to the component across which voltage is measured. By having a very high total resistance, the voltmeter draws minimal current from the main circuit, thus not significantly changing the potential difference it is supposed to measure.
5. How does the concept of an ideal voltmeter relate to this conversion?
An ideal voltmeter has infinite resistance so that it draws zero current from the circuit and measures the true potential difference. By connecting a very high resistance in series with the galvanometer, we make the total resistance of the resulting voltmeter very large. This makes the converted galvanometer behave more like an ideal voltmeter, increasing its accuracy.
6. What is the key difference between converting a galvanometer into a voltmeter versus an ammeter?
The key difference lies in the configuration and value of the added resistance:
- For a Voltmeter: A high resistance is connected in series with the galvanometer.
- For an Ammeter: A very low resistance (called a shunt) is connected in parallel with the galvanometer.
7. How can the range of the converted voltmeter be increased?
To increase the range of the voltmeter (i.e., to measure a higher maximum voltage 'V'), the value of the series resistance 'R' must be increased. According to the formula R = (V/I_g) - G, a larger 'V' requires a larger 'R' to keep the current (I_g) at the full-scale deflection value. Essentially, a higher resistance is needed to drop the larger excess voltage.
8. What would happen if the series resistor used for the conversion has a resistance that is too low?
If the series resistance is too low, the total resistance of the voltmeter will be insufficient. When connected to a circuit to measure a voltage within its intended range, a current larger than the galvanometer's full-scale deflection current (I_g) will flow through it. This will cause the pointer to go off-scale and can permanently damage the sensitive coil of the galvanometer due to overheating.
