Construction and Calibration of Simple Measuring Instruments for SS 1

Construction of Simple Measuring Instruments to Measure Quantities

Performance Objectives

By the end of the lesson, students should be able to:

1. state the units of different physical quantities;
2. construct simple measuring instruments (e.g. electrical continuity tester, voltmeter).

Introduction to Physical Quantities and Measurement

Physics is a science that studies matter, energy, and their interactions. To understand these interactions, it is essential to measure various aspects of the physical world. These measurable aspects are called physical quantities.

A physical quantity is any property of matter or energy that can be measured and expressed with a numerical value and a unit.

Examples of physical quantities include length, mass, time, temperature, and electric current. Each physical quantity has a standard unit that helps in expressing its measured value consistently.

Units of Physical Quantities

For measurements to be meaningful and universally understood, standard units are used. The most widely accepted system of units is the International System of Units (SI), which is based on the older MKS (Metre-Kilogram-Second) system. The SI system defines seven base units from which all other units are derived.

Base SI Units

The fundamental physical quantities and their SI units are:

1. Length: The SI unit for length is the metre (m).
2. Mass: The SI unit for mass is the kilogram (kg).
3. Time: The SI unit for time is the second (s).
4. Electric Current: The SI unit for electric current is the ampere (A).
5. Temperature: The SI unit for temperature is the kelvin (K).
6. Amount of Substance: The SI unit for amount of substance is the mole (mol).
7. Luminous Intensity: The SI unit for luminous intensity is the candela (cd).

Derived SI Units

Many other physical quantities are derived from these base units. Their units are combinations of the base units. Examples of derived quantities and their units include:

1. Area: The unit for area is square metre (m²).
2. Volume: The unit for volume is cubic metre (m³).
3. Speed: The unit for speed is metre per second (m/s).
4. Density: The unit for density is kilogram per cubic metre (kg/m³).
5. Force: The unit for force is the newton (N), which is equivalent to kg·m/s².
6. Energy/Work: The unit for energy or work is the joule (J), which is equivalent to N·m.
7. Power: The unit for power is the watt (W), which is equivalent to J/s.
8. Electric Charge: The unit for electric charge is the coulomb (C), which is equivalent to A·s.
9. Electric Potential Difference (Voltage): The unit for voltage is the volt (V), which is equivalent to J/C.
10. Resistance: The unit for resistance is the ohm (Ω), which is equivalent to V/A.

Understanding these units is crucial for accurate measurement and calculation in physics.

Electrical Continuity Tester

An electrical continuity tester is a simple device used to check if an electrical path is complete between two points. It helps to determine if a circuit is closed (continuous) or open (broken).

Working Principle of the Electrical Continuity Tester

The working principle of an electrical continuity tester is based on Ohm’s Law and the concept of a closed circuit. When the probes of the tester are connected across a continuous path (a conductor), current flows from the power source through the indicator (bulb or LED) and back to the source, completing the circuit. This current flow causes the indicator to light up, showing continuity.

If there is a break in the circuit (an open circuit or a non-conductor), current cannot flow. The indicator remains off, showing a lack of continuity. The device essentially checks for a low-resistance path.

Constructing a Simple Electrical Continuity Tester

Constructing a simple continuity tester involves connecting a power source, an indicator, and probes in series. This allows for easy testing of electrical paths.

The required materials are:

1. A DC power source (e.g., 9V battery or two 1.5V AA batteries in series).
2. An electric bulb (e.g., a small torch bulb) or an LED (Light Emitting Diode) with a suitable current-limiting resistor if using an LED.
3. Connecting wires.
4. Two probes (e.g., alligator clips or sharpened wire ends).
5. Insulating tape or heat shrink tubing.

The procedure involves the following steps:

1. Prepare the components: Ensure all components are in good working condition. If using an LED, connect a resistor (e.g., 220 Ω to 1 kΩ) in series with its positive leg (the longer leg) to prevent it from burning out.
2. Connect the battery: Take one end of a connecting wire and connect it to the positive terminal of the battery.
3. Connect the indicator: Connect the other end of the wire from the battery to one terminal of the bulb holder (or the resistor connected to the LED’s positive leg).
4. Connect the first probe: Take another connecting wire and connect one end to the other terminal of the bulb holder (or the negative leg of the LED). Connect the other end of this wire to one of the probes.
5. Connect the second probe to the battery: Take a final connecting wire and connect one end to the negative terminal of the battery. Connect the other end of this wire to the second probe.
6. Secure connections: Use insulating tape or heat shrink tubing to secure all connections and prevent short circuits.
7. Test the tester: Touch the two probes together. The bulb or LED should light up, indicating that the tester is working correctly.

Using the Electrical Continuity Tester to Detect Faults

Once constructed, the continuity tester can be used to check for breaks in wires, fuses, switches, or other components.

The procedure for detecting faults is:

1. Ensure safety: Always ensure that the circuit or component being tested is disconnected from its main power supply to avoid electric shock.
2. Place probes: Touch one probe to one end of the component or wire being tested and the other probe to the other end.
3. Observe the indicator:
   1. If the bulb lights up (or LED glows), it means there is a continuous path, and the component is working correctly (low resistance).
   2. If the bulb does not light up (or LED does not glow), it means there is a break in the path (high resistance or open circuit), and the component is faulty.
4. Example applications:
   1. Testing a wire: Touch probes to each end of a wire. If the bulb lights, the wire is good. If not, it is broken.
   2. Testing a fuse: Touch probes to each end of a fuse. If the bulb lights, the fuse is intact. If not, it is blown.
   3. Testing a switch: With the switch in the “ON” position, touch probes across its terminals. If the bulb lights, the switch is working. Repeat for the “OFF” position; the bulb should not light.

Galvanometer

A galvanometer is a sensitive instrument used for detecting and measuring small electric currents. It works by deflecting a pointer in response to the magnetic field produced by the current flowing through a coil.

Working Principle of the Galvanometer

The working principle of a galvanometer is based on the motor effect, which states that a current-carrying conductor placed in a magnetic field experiences a force. In a galvanometer, a coil of wire is suspended within a strong magnetic field. When a small electric current passes through the coil, it creates its own magnetic field. The interaction between this magnetic field and the permanent magnetic field causes the coil to rotate.

This rotation is opposed by a spring, which provides a restoring force. The coil rotates until the magnetic torque is balanced by the spring’s restoring torque. A pointer attached to the coil moves across a calibrated scale, indicating the magnitude of the current. The deflection is proportional to the current flowing through the coil.

Voltmeter

A voltmeter is an instrument used to measure the electric potential difference (voltage) between two points in an electrical circuit. It is always connected in parallel across the component whose voltage is to be measured.

Working Principle of the Voltmeter

A voltmeter is essentially a galvanometer connected in series with a very high resistance resistor. The high resistance is crucial because a voltmeter must draw very little current from the circuit it is measuring. If it drew significant current, it would alter the circuit’s behaviour and give an inaccurate reading.

When connected in parallel, the high resistance ensures that most of the current bypasses the voltmeter, flowing through the component being measured. The small current that does flow through the galvanometer causes a deflection proportional to the voltage drop across the high resistance and the galvanometer coil. The scale of the galvanometer is then calibrated to read voltage directly.

Constructing a Simple Voltmeter

A simple voltmeter can be constructed by modifying a sensitive galvanometer. The key is to add a high resistance in series with the galvanometer coil.

The required materials are:

1. A small galvanometer (sensitive enough to detect small currents).
2. A high resistance resistor (e.g., 10 kΩ or higher, depending on the desired voltage range and galvanometer sensitivity).
3. Connecting wires.
4. Two probes (e.g., alligator clips or sharpened wire ends).
5. Insulating tape.

The procedure involves the following steps:

1. Identify galvanometer terminals: Locate the positive and negative terminals of the galvanometer.
2. Connect the high resistance: Connect one end of the high resistance resistor to one terminal of the galvanometer. This resistor is called a “multiplier resistor.”
3. Connect the probes: Connect one probe to the other end of the high resistance resistor. Connect the second probe directly to the remaining terminal of the galvanometer.
4. Secure connections: Use insulating tape to secure all connections.
5. Calibration (conceptual): A simple constructed voltmeter will not have a pre-calibrated scale for voltage. To make it useful, it would need to be calibrated against a known voltage source (e.g., a 1.5V battery, a 9V battery) and markings made on its scale. For SS1, the focus is on the construction and understanding the principle rather than precise calibration. When connected across a voltage source, the galvanometer pointer will deflect, and the extent of deflection will be proportional to the voltage.

When using the constructed voltmeter, remember to connect it in parallel across the component or power source whose voltage is to be measured. The high resistance ensures that the voltmeter does not significantly alter the circuit’s current.