Final answer:
The amplitude of the indication from a discontinuity during an eddy current test depends on the electrical conductivity, the frequency of the magnetic field, the strength of the magnetic field, and the material's geometry and size.
Step-by-step explanation:
During an eddy current test on tubing, the amplitude of the indication from a discontinuity is influenced by several factors. Eddy currents are loops of electrical current induced within conductors by a changing magnetic field in the conductor. These induced currents can produce a significant magnetic damping effect, which is the drag on motion when a metal object moves through a magnetic field.
Specifically, the amplitude of an eddy current and hence the indication of a discontinuity depends on factors such as:
- The electrical conductivity of the material.
- The frequency of the alternating magnetic field.
- The strength of the magnetic field.
- The geometry and size of the material.
Eddy currents create a counterforce to the motion of the conductor which is known as magnetic damping. This is the principle behind their application in braking systems in trains and roller coasters, as the eddy currents produce a significant drag, slowing the motion of the object. Demonstrations with metal pendulum bobs and magnets illustrate this effect: a solid metal bob experiences more drag than a slotted metal plate or an insulator when moving through a magnetic field.