Final answer:
A hydrometer is used to check solution strength in the darkroom by measuring the specific gravity or density of liquids.
Step-by-step explanation:
An easy way to check solution strength in the darkroom is with a hydrometer. A hydrometer is a device used to measure the specific gravity or density of liquids. This is relevant when mixing photographic chemicals for film development because the strength of the solution can affect the development process. The hydrometer floats at a level in the liquid that is dependent on the liquid's density; the denser the liquid, the higher the hydrometer will float. This reading can help ensure solutions are mixed to the correct concentrations for optimal photographic results.