What device is used to change alternating voltage to a higher or lower value?

A transformer is specifically designed to change the voltage level of alternating current (AC) electricity. It operates on the principles of electromagnetic induction and can either increase (step-up transformer) or decrease (step-down transformer) the voltage while maintaining the same power level, assuming no losses. This makes transformers essential in various applications, particularly in power distribution systems and electrical devices where specific voltage levels are required for optimal performance.

The other devices listed serve different functions. An X-ray tube is used to generate X-rays for radiographic imaging, but it does not alter voltage levels directly like a transformer. A betatron is an accelerator that increases the energy of electrons to produce high-energy X-rays but is not primarily intended for voltage transformation. Similarly, a linear accelerator is a device used in radiation therapy to accelerate charged particles, significantly increasing their energy for medical applications, but it does not function as a voltage-changing device. Thus, the transformer is the correct answer for changing alternating voltage values.