In a test coil with primary and secondary windings, what affects the voltage across the secondary?

The voltage across the secondary winding of a test coil in electromagnetic testing is influenced by several factors, making the option that includes all of them the most comprehensive and correct answer.

When considering test material permeability, materials with higher permeability will allow magnetic lines of force to concentrate more effectively, which can increase the induced voltage in the secondary winding. This is because a change in the magnetic field effectively produces a change in the induced voltage according to Faraday's law of electromagnetic induction.

Test frequency also plays an integral role, as the rate at which the magnetic field alternates can affect how much voltage is induced in the secondary. Higher frequencies can lead to higher induced voltages, as they can create a more dynamic magnetic field.

The geometry of the test object is another key factor. The shape, size, and configuration of the test material can impact how magnetic fields interact with the windings of the coil. Variations in geometry can lead to changes in flux linkage between primary and secondary windings, thereby affecting the voltage induced in the secondary.

Considering that all of these factors—material permeability, frequency, and geometry—interact to impact the induced voltage across the secondary winding, the correct answer encompasses all of them, accurately reflecting the complexities involved in electromagnetic testing.