The question asks to identify the effect that is associated only with voltmeters. Let's analyze the common effects used in electrical measuring instruments.

Understanding Effects in Electrical Measuring Instruments

Different physical principles, or effects, are utilized in the design and operation of various electrical measuring instruments like ammeters, voltmeters, and wattmeters. Understanding these effects helps determine which instruments rely on them.

• Thermal Effect: When current flows through a conductor, it generates heat. This heat causes a change in temperature. This principle is used in hot-wire instruments, which can measure both AC and DC current or voltage based on the thermal expansion caused by the heat. This effect is not exclusive to voltmeters as it's also used in ammeters.
• Electro-dynamic Effect: This effect is based on the force between current-carrying conductors. It is used in electrodynamic instruments, which can measure AC or DC current, voltage, or power (as in wattmeters). This effect is not exclusive to voltmeters.
• Chemical Effect: This effect deals with chemical changes caused by the flow of electric current (electrolysis). This principle is primarily related to phenomena like electroplating or the operation of batteries. It is not typically used as the primary operating principle for standard voltmeters, ammeters, or wattmeters used for circuit measurement, though it's involved in processes like electrolytic current measurement (coulometers), which are not standard meters.
• Electrostatic Effect: This effect is based on the force of attraction or repulsion between charged surfaces due to the voltage (potential difference) applied between them. This force can be used to move a pointer against a spring or gravity, indicating the magnitude of the voltage. Electrostatic instruments measure voltage directly by the force exerted by the electric field, without requiring significant current flow. They are particularly useful for measuring high DC or AC voltages because they have very high input impedance. This effect is fundamentally related to voltage and is the operating principle behind electrostatic voltmeters. While other types of voltmeters exist (like moving-coil, moving-iron, thermal), the electrostatic effect is the primary principle behind electrostatic voltmeters, and this effect is uniquely suited for directly measuring voltage based on electric field forces, making it strongly associated with voltage measurement via this specific type of meter.

Analyzing the Association with Voltmeters

We need to find the effect associated only with voltmeters. Let's reconsider the options:

• Thermal effect: Used in both ammeters and voltmeters (hot-wire instruments). Not exclusive.
• Electro-dynamic effect: Used in ammeters, voltmeters, and wattmeters. Not exclusive.
• Chemical effect: Not a primary operating principle for standard voltmeters used in circuits.
• Electrostatic effect: This is the principle behind electrostatic voltmeters, which measure voltage directly using electric forces. While other types of voltmeters exist, the electrostatic effect is fundamentally tied to the concept of potential difference (voltage) creating an electric field and associated forces, and its practical application in metering is predominantly in electrostatic voltmeters. Among the given options, it is the effect most uniquely associated with a specific type of voltmeter and the direct measurement of voltage via electric fields.

Considering the typical applications of these effects in standard measuring instruments, the electrostatic effect is the one most uniquely or primarily associated with a specific type of voltmeter used for measuring voltage based on the forces between charged surfaces due to potential difference.

Based on the analysis, the electrostatic effect is the principle specifically utilized in electrostatic voltmeters and is the effect among the choices that is most uniquely associated with measuring voltage directly through electric forces.