Which of the following is a key characteristic of the Heliograph?

The Heliograph is fundamentally a signaling device that utilizes sunlight to convey messages. It achieves this by reflecting sunlight using a mirror to create flashes of light, which can be seen over long distances. This characteristic is what makes it distinct from other forms of communication devices, such as those that send messages via sound or rely solely on electrical means. The effectiveness of the Heliograph is dependent on clear visibility and the ability to harness sunlight, allowing for communication without the need for any complex electronic components.

While a trained operator may be needed to encode and interpret the flashes of light correctly, this is not what primarily defines the Heliograph itself; rather, it is the method of using sunlight as a medium that defines its core functionality.