What type of transformer allows for continuous adjustability?

A variable transformer is designed specifically to provide continuous adjustability of the output voltage. This is achieved through a mechanism that allows the user to change the turns ratio dynamically while the transformer is in operation. Variable transformers, often referred to as variacs, are used in various applications where precise control of voltage is required, such as in laboratory settings, testing equipment, and power supply regulation.

In such a transformer, the output voltage can be smoothly varied from zero up to the maximum input voltage, making it highly versatile for applications that demand fine-tuning of voltage levels. The adjustability is typically accomplished using a movable contact that slides along a winding, altering the effective number of turns in the circuit.

Other types of transformers mentioned, like isolation, step-up, and step-down transformers, serve more specific purposes that do not include continuous voltage adjustment. An isolation transformer is primarily used to decouple circuits for safety and noise reduction; step-up transformers increase voltage for transmission purposes, while step-down transformers reduce voltage for consumer use. These functions are static and predetermined, meaning they do not offer the flexibility of continuous adjustment like a variable transformer does.