A mercury lamp works by passing an electric arc through mercury vapor, causing it to emit light. This process involves the ionization of gases within the lamp and the subsequent creation of plasma.
The Working Principle
The operation of a mercury lamp begins with a small amount of inert gas, typically argon, and a droplet of liquid mercury sealed within a glass tube.
Here's a step-by-step breakdown based on the mechanism:
- Initiation with Argon: When the lamp is first energized, a voltage is applied between the electrodes. This initial voltage is sufficient to ionize the inert gas (argon).
- Argon Conduction and Heating: As the ionized argon conducts, an initial arc forms. The heat generated from this arc vaporizes the liquid mercury present in the tube.
- Mercury Vaporization: The mercury turns from liquid to gas.
- Mercury Ionization: Next, the increasing voltage between the two main electrodes will ionize the mercury gas.
- Main Arc Formation: Once the mercury gas is ionized, a stronger arc initiates between the two main electrodes. This main arc passes through the mercury vapor plasma.
- Light Emission: The energy from the arc excites the mercury atoms. As these excited atoms return to their normal state, they emit photons. The lamp will then radiate mainly in the ultraviolet, violet and blue emission lines.
This specific light spectrum, predominantly in the ultraviolet and blue regions, is characteristic of mercury vapor discharge. In many practical applications, the inner surface of the lamp tube is coated with a phosphor that absorbs the ultraviolet light and re-emits it as visible light, improving color rendering and efficiency.
Summary Table:
| Step | Action | Result |
|---|---|---|
| Start-up | Voltage applied; Argon ionized | Initial arc forms, generating heat |
| Mercury Vaporization | Heat from argon arc; Liquid mercury absorbs | Liquid mercury vaporizes into mercury gas |
| Main Arc Initiation | Voltage increases; Mercury gas ionized | Strong arc initiates between main electrodes |
| Light Generation | Arc excites mercury atoms | Lamp radiates ultraviolet, violet and blue emission lines |
By utilizing the electrical properties of ionized gases and the unique emission spectrum of mercury, these lamps efficiently convert electrical energy into light.
