In measuring instruments, a particular type of recording instrument is obtained by modifying the construction of the indicating instrument. This is done by replacing the pointer and scale of normal indicating instrument by a ________.

Explanation:

In measuring instruments, a particular type of recording instrument is obtained by modifying the construction of the indicating instrument. This is done by replacing the pointer and scale of a normal indicating instrument by a light arm carrying an ink pen.

Correct Option Analysis:

The correct option is:

Option 3: A light arm carrying an ink pen

This option is correct because the transformation of an indicating instrument into a recording instrument is typically achieved by replacing the traditional pointer and scale with a light arm that carries an ink pen. This ink pen then records the measurements on a continuous paper roll or chart. This method allows for the continuous monitoring and recording of measurements over time, which is essential for various applications, including industrial processes and laboratory experiments.

The use of a light arm carrying an ink pen is advantageous because it provides a clear and permanent record of the data. This record can be analyzed later to understand trends, patterns, or anomalies in the measurements. Furthermore, this setup is relatively simple and cost-effective, making it a popular choice for many recording instruments.

Additional Information:

To further understand the analysis, let’s evaluate the other options:

Option 1: Polarised moving iron instrument

This option is incorrect because a polarised moving iron instrument is not related to the modification of an indicating instrument into a recording instrument. A polarised moving iron instrument is a type of electrical measuring device used to measure alternating currents and voltages. It operates based on the principle of electromagnetic induction, where the movement of a ferromagnetic element (the moving iron) is influenced by the magnetic field generated by the current or voltage being measured. This option does not involve the use of a light arm carrying an ink pen for recording measurements.

Option 2: Clockwork of constant speed

This option is incorrect because a clockwork mechanism of constant speed is not directly related to the modification of an indicating instrument into a recording instrument. While a clockwork mechanism can be used in various instruments to provide a consistent and reliable source of motion or timing, it is not the primary component used to replace the pointer and scale of an indicating instrument for the purpose of recording measurements. The correct approach involves the use of a light arm carrying an ink pen to create a continuous record of the data.

Option 4: Ironless dynamic instrument

This option is incorrect because an ironless dynamic instrument is not relevant to the modification of an indicating instrument into a recording instrument. Ironless dynamic instruments are typically used in applications where high precision and low inertia are required, such as in certain types of electrical measuring devices or sensors. These instruments do not involve the use of a light arm carrying an ink pen for recording purposes.

Conclusion:

In summary, the correct method to modify an indicating instrument into a recording instrument involves replacing the pointer and scale with a light arm carrying an ink pen. This setup allows for the continuous recording of measurements on a paper roll or chart, providing a clear and permanent record of the data. The other options provided do not accurately describe this modification process and are not suitable for creating a recording instrument.