Faraday Cap
Definition and meaning of Faraday Cap in chemistry.
A Faraday cap (more commonly called a Faraday cup) is a hollow, conductive metal container that catches a beam of charged particles, ions or electrons, so the resulting electric current can be measured.
In more detail
When charged particles strike the inner wall, they deposit their charge onto the metal, and an external circuit measures this current, which is proportional to the number of particles arriving per second. The cup is connected through a sensitive electrometer or ammeter, and features like a deep, narrow entrance or a suppressor electrode minimize the escape of secondary electrons that would otherwise skew the reading. Because the response is simple, linear, and traceable to a direct charge measurement, Faraday cups serve as absolute current detectors and are often used to calibrate more sensitive but less quantitative detectors.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Also known as | Faraday cup |
| Measures | Ion or electron beam current |
| Common use | Detector in mass spectrometry and ion beam diagnostics |
In a magnetic-sector mass spectrometer, ions separated by mass-to-charge ratio strike a Faraday cup detector positioned at the end of the flight path; the tiny current generated is amplified to produce the intensity peaks of the mass spectrum.
Frequently asked questions
Why use a Faraday cup instead of an electron multiplier?
A Faraday cup gives a simple, highly linear, absolute measurement of ion current without internal amplification, making it ideal for calibration, though it is less sensitive than an electron multiplier for very weak ion beams.
Is a Faraday cap the same as a Faraday cage?
No. A Faraday cage shields an enclosed space from external electric fields, while a Faraday cup (cap) is a small detector that captures and measures a beam of charged particles.